Add max_databases setting [DDB-1584]

[Khatskevich]

Add max_databases setting

max_databases allows to limit max number of databases a user
can create. In case the user tries to create a new database and
the current number of databases is equal or larger then max_databases
an Error is thrown.

Changelog category (leave one):

• New Feature
• Experimental Feature
• Improvement
• Performance Improvement
• Backward Incompatible Change
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• Critical Bug Fix (crash, data loss, RBAC) or LOGICAL_ERROR
• Bug Fix (user-visible misbehavior in an official stable release)
• CI Fix or Improvement (changelog entry is not required)
• Not for changelog (changelog entry is not required)

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...

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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