Provide a storage backend for dblog module

Problem/Motivation

logger.dblog is backend overrideable but it's pointless to override it because the module is littered with database queries.

Proposed resolution

• Create a storage backend for the dblog module.
• Create a new LogEntry class to avoid the need of interact directly with database records.
• Put all the queries of the dblog.module in this storage.
• Make this storage backend available as a service.
• Refactor the dblog module to interact with this new service
• Delete the use of $SESSION from the controller. This feature is covered by: #2015149: Replace dblog recent log entries with a view

Remaining tasks

• Update this patch removing the views integration changes already introduced here: #2851293: dblog is using the wrong views field, filter and relationships definitionsDone
• Refactor DbLogController::buildFilterQuery() to return a [field, [values, ...], operator] array. That the storage backend can use to build the filter.
• Add test coverage for the refactor of buildFilterQuery
• Refactor DbLogStorage::loadMultiple to use the new filter notation described above.
• Add unit tests for the LogEntry class

User interface changes

None.

API changes

1. A new DbLogStorage service (storage.dblog).
2. A new LogEntry class.
3. _dblog_get_message_types deprecated. CR: https://www.drupal.org/node/2857426