Use Lucene version string in main rest action

This commit removes the deprecated constant for the main
version and uses the real lucene version we are running instead.
Behind the scenes the same value was used and is now obsolet.
Add more trace logging to gateway translog recovery
Core: put []'s around the values in the slow-merge log message
[TEST] Use private store for a test private engine

The test was using the same store as the suite level engine which caused
problems with write locks in some cases.

Closes #8880
[CORE] Drop support for state written pre 0.90

Today we have several upgrade methods that can read state written
pre 0.90 or even pre 0.19. Version 2.0 should not support these state
formats. Users of these version should upgrade to a 1.x or 0.90.x version
first.

Closes #8850