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
If a thread dies without releasing its connection, will this cache hang on to it indefinitely?

How does this compare to using thread-locals directly, like `thread[:"_connpool_#{id}_cached_connection"]` ?