Easier lightweight meshy client usage

[tea-dragon]

It is perhaps desirable to be able to use arbitrary numbers of MeshyClients in limited scope environments without worrying about undue resource expenditure. To this end, MeshyClients (and the client portion of full Servers) can/should probably all use the same netty thread pool and intelligently clean up after themselves -- at least to the extent that it doesn't prevent shutdowns. This probably involves some logic where the client pool will stay up as long as a Server is up, for some time period after the last client was closed, and/or uses an exiting thread pool (from guava).