Expose config option to keep alive TCP connections

[stmcginnis]

This adds a ReuseConnections configuration flag. When set, this will try to configure the HTTPClient transport to not immediately close connections so that they can be reused by subsequent API calls.

Closes: #349

Testing

I ran something that collects a lot of different information from a system. Its original unmodified timing was:

real	0m35.706s
user	0m0.165s
sys	0m0.063s

After setting the new reuse flag, the same operations timing was:

real	0m26.284s
user	0m0.266s
sys	0m0.083s

I also tried tweaking both ReuseConnections: true and MaxConcurrentRequests: 2 and got even better results:

real	0m21.510s
user	0m0.092s
sys	0m0.039s

[stmcginnis]

Local testing is good, so I'm going to merge this. But I would still appreciate any feedback from others to make sure this seems to work as expected and to know if there are any issues with this approach that I've overlooked. Please open an issue if there are any negative side effects from this that I've overlooked.