Thread version (ie. no async code) for gen_server

[ElFantasma]

Initial POC for a OS threaded version of gen_server, instead of using async/await model.

• Both models coexists, and all examples are duplicated to run on both versions.
• Only a module switch and some minor modifications (eg. remove async and await declarations) are required to support one model or the other.

Pending improvements (probably implemented in future tickets):

• Not sure about modules naming: async/sync are not correct names for both versions, maybe they should be something like thread/task models.... Suggestions are welcome
  Edit: Already renamed to threads/tasks as they are more explicit.
• OS Threads should be provided from a pool instead of spawning freely on demand
• Runtime creation to run async code within threaded model should be centralized and reused in case it's required more than once.
• Possibly hide both implementation under feature flags, to enable/disable each one