When sending shard start/failed message due to a cluster state change, use the master indicated in the new state rather than current

This commit also adds extra protection in other cases against a master node being de-elected and thus being null.

Closes #6189
Unified MetaData#concreteIndices methods into a single method that accepts indices (or aliases) and indices options

Added new internal flag to IndicesOptions that tells whether aliases can be resolved to multiple indices or not.

Cut over to new metaData#concreteIndices(IndicesOptions, String...) for all the api previously using MetaData#concreteIndices(String[], IndicesOptions) and removed old method, deprecation is not needed as it doesn't break client code.

Introduced constants for flags in IndicesOptions for more readability

Renamed MetaData#concreteIndex to concreteSingleIndex, left method as a shortcut although it calls the common concreteIndices that accepts IndicesOptions and multipleIndices
Shade mustache into org.elasticsearch.common package

Previously we shared the jar but never rewrote the packages such
that the shading had no effect.

Closes #6192