This intorduces an optimisation of the EQL requests when these target
one remote cluster only (i.e. no mixed local and remote indices or
multiple remote clusters). In this case, the EQL request is forwarded
to the remote cluster and executed there, instead of having the local
cluster perform multiple querries of the remote cluster.

Adress doc review comments. Update error messages

- rephrase docs;
- EqlSearchResponse.Event allows setting the index value;
- update comment.

Also, fix a case of index qualifying in an exception / failure case.

Add de-/serialization of ccs_minimize_roundtrips setting to
action/EqlSearchRequest.