Add more traces in case of failure when testing with actual plugins
remove default `_all` for `type` and `index` if these are missing in REST tests

If a type or path is missing in the REST test yaml file, it is
automatically replaced with _all. This makes it hard to test changes
in the api, for example adding the possibility to leave the index
blank in addition to _all and * in the uri.

closes #4657
[DOCS] Added documentation for SameShardAllocationDecider

Closes #4615
give the compiler more memory, otherwise it fails sometimes
improve buffered output
- use the same buffer size if wrapping a buffered output
- directly call flush on the checksum wrapper