trillium-client-v0.3.1

The focus of this release is on the pool implementation. In particular, we

• use a bounded lock-free queue for concurrent access
• resolve a bug wherein we could have drained a connection for each of the socket addrs that the ToSocketAddrs resolved to, even if only one was used
• provide a limited cleanup entrypoint. currently it only garbage collects completely empty poolsets, but eventually it should iterate over connections in the poolset and test for expiration and liveness
• add tests for standard pool behavior because it is not tested directly in any doctests