Allow slop = -1 in span queries

Closes #3673
Added nested filter join option to the docs.

Closes #3738
Set nowInMillis to search context created by the count api so that "NOW" can be used within queries

Added nowInMillis to ShardCountRequest in a backwards compatible manner

Fixes #3625
Set nowInMillis to search context created by the validate query api so that "NOW" can be used within queries

Added nowInMillis to ShardValidateQueryRequest in a backwards compatible manner

Fixes #3629
Set nowInMillis to search context created by the explain api so that "NOW" can be used within queries

Added nowInMillis to ExplainRequest in a backwards compatible manner

Fixes #3626
don't clear gateways in reroute test
- also remove test logging from percolator
Moved SimpleIndexStateTests to AbstractIntegrationTest
Add/fix index selectors.

Closes #3758.
More AbstractIntegrationTest
Added @TestLogging to all methods to see which shards we refresh and which ones we search on

Increased wait timeout and added multiple iterations there as well, one is not enough. We want to verify whether the problem gets fixed later on.