Summary:
This change introduces a pure virtual initial implementation of the transaction
engine which is then implemented in two versions: single node and distributed.
The interface classes now have the following hierarchy:
```
Engine (pure interface)
|
+----+---------- EngineDistributed (common logic)
| |
EngineSingleNode +-------+--------+
| |
EngineMaster EngineWorker
```
In addition to this layout the `EngineMaster` uses `EngineSingleNode` as its
underlying storage engine and only changes the necessary functions to make
them work with the `EngineWorker`.
After this change I recommend that you delete the following leftover files:
```
rm src/distributed/transactional_cache_cleaner_rpc_messages.*
rm src/transactions/common.*
rm src/transactions/engine_rpc_messages.*
```
Reviewers: teon.banek, msantl, buda
Reviewed By: teon.banek
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D1589
96ece11cdd