Bad test setup as a cause of the race failure

Test setup for checking of replication events processing was incorrect and it lead to failing tests from time to time (especially with a race detector). Global gitay-ruby instance must be used across all tests in the package as its startup is time consuming. Multiple instances of the Gitaly can't be used in the test as they share same global Config and there is intersection that can't be resolved (same internal socket used for all instances). Right order of binaries preparation and config setup. Configuring logs to be json-formatted for better readability. gRPC services must be registered before Serve call is made.

Closes: #2592 (closed)