[Tests] Reduce the number of threads used by the BackgroundIndexer by default

It sometimes overloads the nodes.
Typo in the description for include_in_all

I know this is uber-minor, but I was confused by the phrase "the raw field value to be copied". I assume "is" was supposed to be instead of "to"
Freq Terms Enum
A frequency caching terms enum, that also allows to be configured with an optional filter. To be used by both significant terms and phrase suggester.
This change extracts the frequency caching into the same code, and allow in the future to add a filter to control/customize the background frequencies
Closes #5597