Refresh metastore on Lambda searcher

[rdettai]

Description

Fix outdated metastore issues on the search side

• add configurable polling interval on searcher
• change polling ticker to DELAY to avoid bursts after longer Lambda inactivity

Also:

• adding some docs summarizing all configurable env variables
• made indexer timeout configurable in CDK script to fix the HDFS indexer example work out of the box
• some refactoring/comments on the LazyFileBackedIndex to help readability

How was this PR tested?

Intensive testing on our cloud account.

[rdettai]

This PR is still failing when creating a new index with:

internal error: could not find checkpoint for index `hdfs-logs:XXXXXXXX` and source `_ingest-lambda-source`

Tried to create the _ingest-lambda-source with CreateIndexRequest, but that is not possible because identifiers must match the following regular expression: ^[a-zA-Z][a-zA-Z0-9-_\.]{2,254}$.

This problem is orthognal to this fix so I'm merging this anyway.