Upstream merge
Add Session wrapper
With this patch we can now use gocqlx like:
```
session.Query(`SELECT * FROM struct_table`, nil).Get(&v)
```
instead of (old format):
```
gocqlx.Query(session.Query(`SELECT * FROM struct_table`), nil).Get(&v)
```
Signed-off-by: Michał Matczuk <michal@scylladb.com>
Depending on what is set, schema agreement is checked before each file or before each statement.
Awaiting schema agreement after every migration has run is always done.
