Test-server per-request configuration: add an option to clear values

Currently the test server will apply per-request configuration specified in per request-header to the process-level configuration, by performing a proto-level `Merge()`.
This doesn't always work well: it's not possible to override the server configuration with type-specific defaults. For example, response-size (int, default=0) cannot be overridden to 0 when the server-level configuration is non-zero.
A bool-valued field called `clear` could be helpful here, to allow the client to indicate it wants to fully specify the configuration (and not inherit from what the server has configured).