Fix erronous pool timeout case

[tomchristie]

Refs #550.

Fixes an issue where a pool timeout can occur, even though a request has an assigned connection.

It's not clear to me if this is a complete resolution, or if something more substantial (see #653) is actually required.
But either way around I think this resolves an edge case for pool timeout: 0 cases.

Needs a test case.

[nihilSup]

@tomchristie can you use two tests from #653? test_pool_under_load and test_pool_timeout_connection_cleanup to be more specific. Maybe I could add them myself but I don't know how

[tomchristie]

I've revised this in #688.
Added failing test cases first in the first commit, then resolved them in the second.