[ZEPPELIN-4504] pre-index notebooks in the background to fix search ability

[amakaur]

What is this PR for?

Before this PR zepp was unable to search for keywords in existing notebooks that have not been updated in any way after a restart.

This PR pre-indexes all of the notebooks at startup time on a separate thread so that zepp is able to find keywords in existing notebooks whenever user uses the search bar.

What type of PR is it?

Bug Fix

What is the Jira issue?

https://issues.apache.org/jira/browse/ZEPPELIN-4504

How should this be tested?

• Create a notebook using Spark interpreter
• add small piece of code such as "println("hello")
• Restart zeppelin
• Search for a keyword "println"

Screenshots (if appropriate)

Questions:

• Does the licenses files need update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[zjffdu]

@amakaur We should first try to figure out why the indexing is lost after restart. Is it due to the index in memory or other bugs ? Reindexing works, but it is some kind of workaround, not an elegant solution for this issue.

[amakaur]

@zjffdu in the previous versions pre-index was done at startup time so looks like in 0.9.0 pre-index ONLY happens when any kind of update happens to a note.