BUG/MINOR: server: fix server_find_by_name() usage during parsing

BUG/MINOR: server: fix server_find_by_name() usage during parsing

Since below commit, server_find_by_name() now search using
'used_server_id' proxy backend tree :
  4bcfe30414005323a8d4ab986bce92bb736b59df
  OPTIM: server: eb lookup for server_find_by_name()

This introduces a regression if server_find_by_name() is used via
check_config_validity() during post-parsing. Indeed, used_server_id tree
is populated at the same stage so it's possible to not found an existing
server. This can cause incorrect rejection of previously valid
configuration file.

To fix this, servers are now inserted in used_server_id tree during
parsing via parse_server(). This guarantees that server instances can be
retrieved during post parsing.

A known feature which uses server_find_by_name() during post parsing is
attach-srv tcp-rule used for reverse HTTP. Prior to the current fix, a
config was wrongly rejected if the rule was declared before the server
line.

This should not be backported unless the mentionned commit is.