#6081: set IO throttling back to 20 MB/sec now that #6018 is fixed
Change key to keyed
fixed MetaData#concreteIndices to throw exception with a single index argument in case allowNoIndices == false and ignoreUnavailable == true

Closes #6137
improved IndicesOptions javadocs
Fix for benchmark test timeout

Lower number of random requests generated for each test so as not to
timeout on heavy tests.

Addresses #6094