MEDIUM: proxy: no longer allocate the default-server entry by default

MEDIUM: proxy: no longer allocate the default-server entry by default

The default-server entry used to always be allocated. Now we'll postpone
its allocation for the first time we need it, i.e. during a "default-server"
directive, or when inheriting a defaults section which has one. The memory
savings are significant, on a large configuration with 100k backends and
no default-server directive, the memory usage dropped from 800MB RSS to
420MB (380 MB saved). It should be possible to also address configs using
default-server by releasing this entry when leaving the proxy section,
which is not done yet.