Simplify text
Locking is not an issue with memory hashtable data.

Locks only matter for drivers that can be accessed from different
processes.
phpdoc
[mms] Add null driver.
Use in-memory hashtable if none configured
Revert "Fix history access when no hash table is configured."

This reverts commit 37c8d463e562956f6a8f360bcd2cb64a82dd8fea.

HashTable factory now always returns a valid object.