MINOR: server: move idle tree insert in a dedicated function

MINOR: server: move idle tree insert in a dedicated function

Define a new function _srv_add_idle(). This is a simple wrapper to
insert a connection in the server idle tree. This is reserved for simple
usage and require to idle_conns lock. In most cases,
srv_add_to_idle_list() should be used.

This patch does not have any functional change. However, it will help
with the next patch as idle connection will be always inserted in a list
as secondary storage along with idle/safe trees.