Previously ecto_sql used pre-defined cache_statement value for all
types of queries. This commit adds ability to use user specified
cache statement as it could improve performance in certain situations
like multiple `insert_all` queries with fixed size batch and so on.

Besides this the commit adds length of parameters to the default
cache_statement name. In other way set of queries like this:

    App.Repo.insert(%MyModel{name: "test_1", field_id: nil})
    App.Repo.insert(%MyModel{name: "test_2", field_id: "test"})
    App.Repo.insert(%MyModel{name: "test_3", field_id: nil})

will lead to situation when the previous prepared statement will be
closed when next query from the set will be executed and new one will
be created because `nil` values are not get into resulted query.