Add debug logging to gRPC entrypoints

[LarryRuane]

This additional logging may help with debugging lightwalletd clients as it may isolate the problem to the client or server (lightwalletd) side.

To see entrypoint logging, you must start lightwalletd with the option --log-level 5 (which is "debug"; the default is 4, which is "info")).

This option shouldn't be used in normal operation, because clients could flood the log, causing crashes due to running out of disk space. Only use this when actually debugging or testing.

For most gRPCs, also log return values if the log-level is 6 ("trace"). Return value logging isn't done if there is an error (it's assumed that the client will log something) or if the output might be very large (we don't want to overrun the log file).

Enabling trace (--log-level 6) enables everything below that level, so entry logging is enabled as well.

Closes #492.