fix: Change from channels to use a Semaphore

[garrensmith]

When under very heavy load, the conn_requests can include to many Sender's with dropped Receivers. When the Conn is passed to a waiting Receiver and it was recently dropped the Conn could be lost causing a mis-count on the num_open stat which stops any new connections from being created and then mobc deadlocks.

This fixes it by switching from a oneshot channel and queue to using a Semaphore to keep track of available connections for any requests.

This fixes https://github.com/importcjj/mobc/issues/63

[garrensmith]

I'll leave this PR open in case anyone else comes across this. We will be managing a forked mobc with this fix over here https://github.com/prisma/mobc

[garrensmith]

Closed based on #69