[ML] Support rollover of state index

[elasticmachine]

Original comment by @dimitris-athanasiou:

This issue is to track work necessary for supporting rolling the state index. We can add tasks as we identify them.

• Add read/write aliases
• For quantiles, check for existing and write to the same index any old quantiles are in
• For data frame analytics progress, check for existing and write to the same index any old progress documents are in
• For persistent state document, check for existing and write to the same index any old documents are in
• Make sure that all get requests for state documents are replaced with ID query _search requests that ignore duplicates
• Make sure all deletes can handle multiple indices
• Make sure index template has a wildcard so new state indices get the template
• Use ILM or other mechanism to create new state indices when the current one reaches a certain size
• Remove empty state indices during nightly maintenance

[droberts195]

@benwtrent this issue lists the work required to make it possible for ML to work when there is more than 1 state index, so is somewhat related to state index upgrade. If you find any other changes will be required when you search for uses of the state index in our code please add them to this issue.

[dimitris-athanasiou]

Note that the persister in ml-cpp expects an index but the CSingleStreamDataAdder does not use the argument. In general, we should maintain control of the index in the java side; c++ should not know which index we're storing the state into.

[droberts195]

c++ should not know which index we're storing the state into

We checked this and currently Java is controlling the index that the state is written into. However, the C++ has constants that contain .ml-state which is confusing, and these should be removed. Currently the C++ "index" variables are just used for unit tests that persist to a directory instead of to Elasticsearch. The "index" is the directory name. But this should be renamed to something like "dummy_index_for_testing" to avoid confusion.

[przemekwitek]

ILM policy for .ml-state* is implemented in master and 7.x.
I'm starting work on the last missing piece i.e. removing empty .ml-state* indices.