UserAPI read only mode

[hg-ms]

We’d like to provide a Microstream operation mode that rejects all store operations trigged by the public store API to prevent accidental changes to the persisted data by applications. It is not intended to prevent all write operations! Write operations by other (internal) APIs should still be possible.

-> The read only mode should prevent write operations triggered via the public storing API from the application code.
-> Write operations trigged during the startup (e.g.: Persisted Roots updates) are still possible.
-> Housekeeping tasks are still possible.
-> Garbage Collection task are still possible.
-> Importing (binary) data must be possible.
-> The read-only mode can be enabled/disabled at runtime (no restart of the storage!)

[hg-ms]

A first solution is available in branch #427

[hg-ms]

merged with PR #428