Cleaner api contracts

[maypok86]

Clarification and motivation

Now otter uses an api very similar to other go cache libraries, but unfortunately it is not particularly beautiful and extensible.

For example, now we have Set(key, value) and SetWithTTL(key, value, ttl) functions, but if we want to add SetIfAbsent(key, value) function, we also need SetIfAbsentWithTTL(key, value, ttl) function, which doesn't look nice anymore. Especially since when we create the cache in the builder we already know its further behavior and whether it will need ttl or not.

We would like to create a cache with a clean api and with the functions it needs using parameters in the builder. This will allow to provide such api as Set(key, value, ttl) and SetIfAbsent(key, value, ttl). Also, since in most cases ttl is the same for all entries in the cache, we don't need to pass it every time at all, further simplifying the api. This will also allow us to use optimizations for the expiration policy, since with the same ttl we only need to maintain a simple fifo queue of items and check their expiration one by one.

Acceptance criteria

1. A more advanced version of the builder has been implemented
2. Otter provides simpler api contracts
3. Implementation tests are written