Log the request metadata in the Execution service

Description

This prints the fields set in RequestMetadata messages in the remote execution service logs.

Changes proposed in this merge request:

• Remove the tool_name and tool_version attributes from Peer to handle metadata at the request instead of session level.
• Add a new server.request_metadata_utils.printable_request_metadata() helper function
• Print the metadata's contents in the debug-level log messages in each execution service servicer functions

Sample output:

2020-05-14 14:26:01,532:[        buildgrid.server.cas.service][DEBUG][gRPC_Executor_0]: FindMissingBlobs request from [ipv6:[::1]:48510] ([tool_name="recc", tool_version="e025109", action_id="", tool_invocation_id="hostname-1023:13256", correlated_invocations_id=""])

Issues addressed

Closes #224 (closed)