Trace Processor: cleanup RPC interface

Minor cleanups to the RPC interface for the --httpd feature.
- Makes the RPC vs original TraceProcessor interfaces more
  uniform and code-search friendly.
- Adds a Get/SetCurrentTraceName() method pair.
- Adds the ability to the Rpc class to adopt an existing instance.
  This will be used for trace_processor_shell -D trace_from_cmdline.
- Makes method naming more consistent.
- Moved trace_processor.proto definitions within the
  perfetto.trace_processor namespace

Bug: 143074239
Change-Id: I29e5b6f5216c3a10afbe036c60f758cbe5dfa172