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. Related: #14891, #14919.

As per @matthewd's suggestion:
 * remove synchronization from around the `@reserved_connections` cache -> ca85c48
Added `test_concurrent_connection_establishment`, though its implementation is kinda hairy :innocent:.