Separate cache clock

[asmodehn]

This is meant to address #29 with the aim to solve #18
We want functional libraries in apps:

• a clock library that tracks the remote clock of a server, simulating time progression, with the same interface as the system clock...
• a cache library that provides TTL-based caching (leveraging nebulex or not). The clock can be the system clock or the one from the clock library. The ttl can be determined based on cached data if it contains some (local or remote) timestamp field already...

[asmodehn]

Note : because we add livebook dependency for documentation, we increase elixir requirement to current 1.14