Parallel Dumps

[brendon]

As requested by @deitch. Investigate dumping databases in parallel to make the most of system resources.

• Respect max-connections with a buffer for other application connections.
• Allow the maximum number of threads to be configured.
• Perhaps base the limit on the number of CPU cores and max-connections in MySQL, though cores can be hard to detect in virtual environments I've been told.

[deitch]

#465 creates parallelism, as well as tests for the actual number of connections.

We can iterate on it later to do things like determining max-connections or other algorithms.

[brendon]

Great work @deitch! :)