Fixes #14891 database connection pool performance regression

Replaces the new Rails 4 connection pool with an old, tried and tested
connection pool called GenePool, yet maintains backward compatibility.

Addresses significant slow down when multiple threads attempt to checkout
connections at the same time. Would take as much as 3 seconds to get an
existing database connection with 100 concurrent threads.

Adds new database connection pool configuration option +checkout_warning+
which logs a warning when it takes longer than the supplied number of
seconds to return a connection from the pool.