Refactor Diametric::Persistence to be primary module

[avescodes]

Refactor Diametric::Persistence such that it becomes the primary module to include and the connection determines different behavior.

A suggestion from @yokolet:

.connect should take an argument :rest or :peer, #peer? and #rest? methods should be on the connection object

[avescodes]

Starting on this issue.

[yokolet]

Originally, Sam suggested this.

Currently, we need to specify peer or rest connection explicitly and define a model individually. But, it is possible to recognize which one is used from url.

The suggestion is putting two persistence models into one. That means we need Diametric::Persistence module only.