Merge pull request #344 from sshcherbakov/master

Fix #294: Thread-safe HashMap to avoid faulty Consumer notifications
#345 Adds the option to configure an NioEventLoopGroup on all Netty clients and servers (both TCP and UDP) and share resources among all started clients and servers.
#345 Add logic to shutdown methods to check before shutting down an EventLoopGroup. If it was passed in with the options, then don't shut it down because we assume it's being shared with other servers/clients. If not, shut down normally.
Introduce jsr166e backport library to use JDK 8's implementation of ConcurrentHashMap which doesn't use synchronization to address #294.