BUG/MAJOR: server: fix stream crash due to deleted server

BUG/MAJOR: server: fix stream crash due to deleted server

Before a dynamic server can be deleted, a set of preconditions must be
validated to ensure it is not referenced naymore by a stream or a
connection. This is implemented in srv_check_for_deletion().

The various criteria specified were incomplete. This allows a server
instance to be deleted while still be referenced by a stream and a
connection.

This bug was reproduced by using ASAN compilation. A script was used to
add and delete a server every second, while using h2load to generate
traffic with download of 1k objects. Here is the ASAN error.

==140916==ERROR: AddressSanitizer: heap-use-after-free on address 0x520000020080 at pc 0x63cb25679537 bp 0x701529ff5070 sp 0x701529ff5060
READ of size 1 at 0x520000020080 thread T7
    #0 0x63cb25679536 in objt_server include/haproxy/obj_type.h:99
    #1 0x63cb2568f465 in process_stream src/stream.c:1823
    #2 0x63cb25a4a4a2 in run_tasks_from_lists src/task.c:632
    #3 0x63cb25a4bf62 in process_runnable_tasks src/task.c:876
    #4 0x63cb2596a220 in run_poll_loop src/haproxy.c:3050
    #5 0x63cb2596b192 in run_thread_poll_loop src/haproxy.c:3252
    #6 0x701539aa9559  (/usr/lib/libc.so.6+0x8b559) (BuildId: c0caa0b7709d3369ee575fcd7d7d0b0fc48733af)
    #7 0x701539b26a3b  (/usr/lib/libc.so.6+0x108a3b) (BuildId: c0caa0b7709d3369ee575fcd7d7d0b0fc48733af)

To fix this, add <curr_used_conns> to the counters checked in
srv_check_for_deletion().

Outside of this bug, one case which remains sensible is for SF_DIRECT
streams which referenced a server instance early in process_stream()
before connect_server(). This occurs with use-server directive,
force-persist rule or cookie persistence. However, after code
reexamination, the code is considered reliable as process_stream() is
not rescheduled before connect_server() invocation. These observations
have been saved in sess_change_server() documentation to ensure it
remains valid in the future.

This must be backported up to 2.6.