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