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 -> 952499b
 * move connection establishment outside of the `synchronized` section -> df7ee38

This should allow for more concurrency. Related: #14891, #14919.

As per @matthewd's suggestion:
 * remove synchronization from around the `@reserved_connections` cache -> 7afd64d
Appears that this method is only ever called with the current thread as default arg. Why does it take an argument? Compare to e.g. `release_current_thread_connection`