Added ExecCAS, ExecCASRelease, GetCAS, GetCASRelease functions
suitable for INSERT ... IF NOT EXISTS and UPDATE's containing IF statement.
Functions returns information wheter query was applied or not, together
with pre-image.
Fixes#98
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>
Releasing query objects in Get/Select could easily lead to
double-releases, which can cause dangerous and tricky data races.
Remove the query field of Iterx and its usages (all release-related).
This is a breaking API change, because it removes the exported method
ReleaseQuery.
Update documenting examples to demonstrate the deferred query release
pattern clients can use to manage query release.
