This patch allows a task to be queued when a backend reaches its
max_connections.  The task will queue on the backend and wait for a
connection to become availble, rather than immediately failing.

This initial commit just adds the basic functionality.  It temporarily
uses the connect_timeout as the queue wait time, until new parameters
are added in followup effort.

The following parameters have been added:
    the amount of time a task will wait.
    the maximum number of tasks that can wait.

    - global parameters:
        backend_wait_timeout  (default 0.0)
        backend_wait_limit    (default 0)

    - those parameters can be overridden in the backend:
        backend foo {
            .host = "bar.com";
            .wait_timeout = 3s;
            .wait_limit = 10;
        }

The backend wait queue capability is off by default and must be
enabled by setting both of the new parameters defined above.

Note that this makes an ABI breaking change.

These counters were added to main:
backend_wait - count of tasks that waited in queue for a connection.
backend_wait_fail - count of tasks that waited in queue but did not
    get a connection (timed out).

As suggested by Nils

This makes sure that we won't abort a backend connection
attempt if the backend can take it. It covers for any
potential missing connwait_signal call.