Use Recovery Throtteling by default.

To prevent to extensive resource use during recovery we use
recovery throtteling by default to prevent unexpected peak load
on clusters. The default is set to 20 MB/sec.

Closes #3035
Use Merge Throtteling by default on node level.

Merge Throtteling is one of the most recommended settings and crucial in the
RealTime indexing case. We should set the default to a reasonable setting
that allows folks to index in a production index and don't see large merge
peaks by default. The default is set to 20 MB/sec on the node level.

Closes #3033
Raise search threadpool default size.

The default size used to be 2x availableProcessors which seemed to
be a to lowish value in practice. 3x appeared to be a sweetspot for
most application. The default is now 3 x availableProcessors

Closes #3023