Improve RpcProviderRegistry loading

Profiling monitor usage at startup has shown that we spend about 19s of
CPU time over ~50 second bootup waiting in getRpcService(). Instead of a
home-grown cache, let's use a guava Loading cache, which allows us to
concurrently instantiate multiple rpc services at the same time.

Change-Id: Ieaaedd7da61cd0b03b05a44ee2a6285bc8eb46ac
Signed-off-by: Robert Varga <rovarga@cisco.com>