#6120 Remove SerialMergeScheduler (master only)

It's dangerous to expose SerialMergeScheduler as an option: since it only allows one merge at a time, it can easily cause merging to fall behind.

Closes #6120
#5882: fix docs
#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
Fix for hanging aborted snapshot during node shutdown

If a node is shutdown while a snapshot that runs on this node is aborted, it might cause the snapshot process to hang.

Closes #5958
[DOCS] Added "Aggregation" to all aggs titles