Support rbtrace and ObjectSpace via environment flags

rbtrace and ObjectSpace are essential to debugging memory
issues in Ruby. Both will be disabled by default.

1. rbtrace allows us to send commands (e.g. request a heap dump) to an
already running gitaly-ruby process. It can be used safely in production.

2. ObjectSpace: Enabling object allocations tracing will allow us to see
what is consuming a significant amount of memory.