Allow selecting local bind address for clients

[daurnimator]

This implementation just passes 'bind' field directly through to cqueues.

cqueues accepts either an address as a string (e.g. .bind = "127.0.0.2") or a table with fields .address and .port (as well as a few aliases).
The latter table form would cause issues with connection pooling, and I have questions about what req:clone() would do (deep copy?).
To avoid such tricky questions, I'm restricting bind to be a string of address:port. where for an ipv6 address you can write [::]:port

[coveralls]

Coverage increased (+0.006%) to 88.043% when pulling e4c8cb6 on pick-local-port into 51e37ff on master.

[coveralls]

Coverage decreased (-0.1%) to 87.909% when pulling 35ce84b on pick-local-port into 51e37ff on master.