Allow users to configure request queue length

[vishalbollu]

Description

Currently, when a waitress queue has reached its connection_limit (default 100), the following is returned: no healthy upstream. Make connection_limit configurable to allow users to customize the length of the queue.

Additional Context

• May need to expose functionality to configure settings such as connection-idle-timeout in waitress, ELB and other network hops so that these networks don't kill requests while they are waiting in a queue.

[nickwalton]

This is super critical for us. So if this can be done as soon as possible would be extremely helpful.