MEDIUM: migrate the patterns reference to cebs_tree

MEDIUM: migrate the patterns reference to cebs_tree

cebs_tree are 24 bytes smaller than ebst_tree (16B vs 40B), and pattern
references are only used during map/acl updates, so their storage is
pure loss between updates (which most of the time never happen). By
switching their indexing to compact trees, we can save 16 to 24 bytes
per entry depending on alightment (here it's 24 per struct but 16
practical as malloc's alignment keeps 8 unused).

Tested on core i7-8650U running at 3.0 GHz, with a file containing
17.7M IP addresses (16.7M different):

   $ time  ./haproxy -c -f acl-ip.cfg

Save 280 MB RAM for 17.7M IP addresses, and slightly speeds up the
startup (5.8%, from 19.2s to 18.2s), a part of which possible being
attributed to having to write less memory. Note that this is on small
strings. On larger ones such as user-agents, ebtree doesn't reread
the whole key and might be more efficient.

Before:
  RAM (VSZ/RSS): 4443912 3912444

  real    0m19.211s
  user    0m18.138s
  sys     0m1.068s

  Overhead  Command         Shared Object      Symbol
    44.79%  haproxy  haproxy            [.] ebst_insert
    25.07%  haproxy  haproxy            [.] ebmb_insert_prefix
     3.44%  haproxy  libc-2.33.so       [.] __libc_calloc
     2.71%  haproxy  libc-2.33.so       [.] _int_malloc
     2.33%  haproxy  haproxy            [.] free_pattern_tree
     1.78%  haproxy  libc-2.33.so       [.] inet_pton4
     1.62%  haproxy  libc-2.33.so       [.] _IO_fgets
     1.58%  haproxy  libc-2.33.so       [.] _int_free
     1.56%  haproxy  haproxy            [.] pat_ref_push
     1.35%  haproxy  libc-2.33.so       [.] malloc_consolidate
     1.16%  haproxy  libc-2.33.so       [.] __strlen_avx2
     0.79%  haproxy  haproxy            [.] pat_idx_tree_ip
     0.76%  haproxy  haproxy            [.] pat_ref_read_from_file
     0.60%  haproxy  libc-2.33.so       [.] __strrchr_avx2
     0.55%  haproxy  libc-2.33.so       [.] unlink_chunk.constprop.0
     0.54%  haproxy  libc-2.33.so       [.] __memchr_avx2
     0.46%  haproxy  haproxy            [.] pat_ref_append

After:
  RAM (VSZ/RSS): 4166108 3634768

  real    0m18.114s
  user    0m17.113s
  sys     0m0.996s

  Overhead  Command  Shared Object       Symbol
    38.99%  haproxy  haproxy             [.] cebs_insert
    27.09%  haproxy  haproxy             [.] ebmb_insert_prefix
     3.63%  haproxy  libc-2.33.so        [.] __libc_calloc
     3.18%  haproxy  libc-2.33.so        [.] _int_malloc
     2.69%  haproxy  haproxy             [.] free_pattern_tree
     1.99%  haproxy  libc-2.33.so        [.] inet_pton4
     1.74%  haproxy  libc-2.33.so        [.] _IO_fgets
     1.73%  haproxy  libc-2.33.so        [.] _int_free
     1.57%  haproxy  haproxy             [.] pat_ref_push
     1.48%  haproxy  libc-2.33.so        [.] malloc_consolidate
     1.22%  haproxy  libc-2.33.so        [.] __strlen_avx2
     1.05%  haproxy  libc-2.33.so        [.] __strcmp_avx2
     0.80%  haproxy  haproxy             [.] pat_idx_tree_ip
     0.74%  haproxy  libc-2.33.so        [.] __memchr_avx2
     0.69%  haproxy  libc-2.33.so        [.] __strrchr_avx2
     0.69%  haproxy  libc-2.33.so        [.] _IO_getline_info
     0.62%  haproxy  haproxy             [.] pat_ref_read_from_file
     0.56%  haproxy  libc-2.33.so        [.] unlink_chunk.constprop.0
     0.56%  haproxy  libc-2.33.so        [.] cfree@GLIBC_2.2.5
     0.46%  haproxy  haproxy             [.] pat_ref_append

If the addresses are totally disordered (via "shuf" on the input file),
we see both implementations reach exactly 68.0s (slower due to much
higher cache miss ratio).

On large strings such as user agents (1 million here), it's now slightly
slower (+9%):

Before:
  real    0m2.475s
  user    0m2.316s
  sys     0m0.155s

After:
  real    0m2.696s
  user    0m2.544s
  sys     0m0.147s

But such patterns are much less common than short ones, and the memory
savings do still count.

Note that while it could be tempting to get rid of the list that chains
all these pat_ref_elt together and only enumerate them by walking along
the tree to save 16 extra bytes per entry, that's not possible due to
the problem that insertion ordering is critical (think overlapping regex
such as /index.* and /index.html). Currently it's not possible to proceed
differently because patterns are first pre-loaded into the pat_ref via
pat_ref_read_from_file_smp() and later indexed by pattern_read_from_file(),
which has to only redo the second part anyway for maps/acls declared
multiple times.