add search-based condition to rule-based automod

[danthe1st]

This PR adds an automod condition to the rule-based automod: The user must have no old enough messages for the automod to trigger. To do this, it first checks the message cache and then uses the new search API (which requires a JDA update) in order to check for older messages.
This message rule is only executed if all previous checks pass (to prevent doing these API requests when it isn't necessary).

As this operation may make a long-running API request, the message filters were changed to run asynchronously.

It also improves the message cache in the following ways:

• Synchronization clears all rows in the message cache and recreates them in a transaction (the database is only used for restoring the cache on startup).
• The message cache is being made thread safe.