Add debug gRPC tracing - logging

[LarryRuane]

There should be a way, either by command-line option or at runtime, to enable logging of gRPC calls. This idea is inspired by #491 in which it's unclear if the stuck zecwallet-lite instance is issuing gRPC calls that are not being handled correctly, or if it's not making any calls (or making incorrect calls).

This would also be useful for testing and development of lightwalletd clients.

We wouldn't want this enabled by default, because doing so would represent a denial of service (DoS) vulnerability, as a client could fill our log file with nonsense calls. This mode should not be preserved across lightwalletd restarts. If this mode is enabled dynamically (using a debug gRPC call), there should also be a way to disable it.