[ENGINE] Wait until engine is started up when acquireing searcher

Today we have a small window where a searcher can be acquired but the
engine is in the state of starting up. This causes a NPE triggering a
shard failure if we are fast enough. This commit fixes this situation
gracefully.

Closes #7455
Docs: typo?

Closes #7458
[SNAPSHOT] Add BWC layer to .si / segments_N hashing

Due to additional safety added in #7351 we compute now a strong hash for
.si and segments_N files which are compared during snapshot / restore.
Old snapshots don't have this hash which can cause unnecessary copying
of large amount of data. This commit adds the ability to fetch this
hash from the blob store if needed.

Closes #7434
Test: mute test until we know what is going on
[Test] rewrite testNoMasterActions to use latest tooling

The test's timeout checks were thrown off by a client created randomly (when the timer was running).

Closes #7432
Tests: Fix FileBasedMappingsTests by using the mappings API instead of field mappings.
Tests: Temporarily ignore SnapshotBackwardsCompatibilityTest