Reducing AR::ConPool's critical (synchronized) section

This is an alternative (much lighter in scope) PR to #14926, only the smallest possible amount of code is changed.

The objective is to:
 * move connection verification outside of `synchronized` section -> 3590eea
 * move connection establishment outside of the `synchronized` section -> 6eda088

This should allow for more concurrency (gist with benched concurrency improvements: https://gist.github.com/thedarkone/80f78dea697f2a7f4150). Related: #14891, #14919.

As per @matthewd's suggestion:
 * remove synchronization from around the `@reserved_connections` cache -> ca85c48
:+1: