[Test] Speed up RandomAllocationDeciderTests
[Test] Speedup RecoveryWhileUnderLoadTests to run with less documents unless tests.nightly=true
[Test] Run intensive suggest tests only if nightly=true
[Test] Speeup FieldData tests and remove unnecessary @Repeat annotations
[Test] Speedup UpdateTests#testConcurrentUpdateWithRetryOnConflict
Add a BackgroundIndexer class to RecoveryWhileUnderLoadTests and use it.

Also change the document distribution a bit between the tests
[Test] allow tests to reuse a singel index across tests

This commit speeds up aggregations tests dramatically since they all
rely on the same index structure. Here we can safe a large amout of time
to not recreate the index for each small test.
Moved BackgroundIndexer to a top level class. Moved waitNumDocs to ElasticsearchIntegrationTestBase.

Update RelocationTests to use the above and unified testPrimaryRelocationWhileIndexing & testReplicaRelocationWhileIndexingRandom into a single randomized test.
[BUILD] Allow to set tests memory via the commandline