Move to a fully declarative model

[sgotti]

To implement features like #144 #157 #159 some breaking changes in the current store model and also in keepers local db file handling are probably inevitable.

Use this breaking point to also move the store model to a full declarative model (similar to the k8s one).
There won't be a "real" api server like in k8s since it'll require implementing it inside the sentinels (to avoid adding additional processes) and making them a frontend between the clients and the store (this will require secure http calls, authentication, need to load balance calls to them, retry if a sentinel doesn't work and many other issues to handle). Instead the store writers (sentinels and stolonctl) will directly generate and validate the "api" data and write them to the store.

This will ease implementation of the above features, remove some overlapping and confusing logics and make future improvements easier to implement.

[sgotti]

Implemented in #178