ALTS: release handshaker service channel if it is not used.

[jiangtaoli2016]

In current ALTS java implementation, we create a shared ALTS thread and a gRPC channel to the handshaker service when we build ALTS channel. The channel to the handshaker service will already be there until application exit. It is more efficient if we can ref count the channel and free this gRPC channel if nobody is using it.

Note that ALTS C++ code does the same resource management: creates a thread and a channel to the handshaker service and only destroys them during grpc shutdown. https://github.com/grpc/grpc/blob/master/src/core/tsi/alts_transport_security.cc#L53

[ejona86]

This isn't about efficiency. If we do not shut down the threads, then in certain environments we will leak class loaders and cause memory leaks.

Also, we should only create the extra threads/channel when they will be used. Creating threads is a tricky business and constantly causes problems. Avoiding it when unnecessary reduces user problems.

There is an efficiency argument as well though: since most users won't use this ALTS code path, we also wouldn't want to waste the resources for everyone for a (currently-)minority use-case.

[zpencer]

@jiangtaoli2016 any updates on this one?

[srini100]

@jiangtaoli2016, can this issue be closed since #5210 is merged?

[jiangtaoli2016]

Yes, Let's close this.