docs: add pgp key to repositories page
Internal: allow InternalEngine to be stopped and started

Once the current engine is started you can only close it once. Once closed the engine cannot be started again. This commit adds a stop method which signals the engine to free it's resources but in a way that allows restarting.

This is done by introducing InternalEngineHolder which is a wrapper around InternalEngine. This allows to add the stop() method without adding complexity the engine implementation. InternalEngineHolder also serves an entry point for listeners (incoming and outgoing) to other ES components, which removes the needs add/remove them if the engine is stopped.

Closes #8784
[ENGINE] Add engine lifecycle store reference to EngineHolder

This commit add the engines reference to the store out of the actual
implementation into the hodler since the holder manages the actual lifcycle.
Engine internal references like per searcher or per recovery are kept inside
the actual implemenation since the have a different lifecycle.
[TEST] Beef up InternalEngineTest and remove bogus timeouts
Docs: Update repositories.asciidoc

1. Enable the repository using "add-apt-repository" to avoid this error "No command 'deb' found".
2. Adding "sudo" to update and install command.

Closes #8691
Docs: Fix the wording for inner hits a bit

Closes #8747
[TEST] LoggingListener to restore the initial logger levels after any modification

Modifications to LoggingListener pushed with #8820 caused the original logger levels not to be reset after modifications, as the new state was saved for restore instead of the previous one.

 Added unit tests for LoggingListener as well.

Closes #8845
[TEST] Restore test logging level after test is done
Core: also log byte size and doc count when we log slow merges

Closes #8855

Closes #8853
Add more trace logging to gateway translog recovery