Refactor the PageCacheMonitor interface to be able to probe much deeper.
Reimplement by-default disabled monitoring of pin/unpin.

Enabling monitoring of pin and unpin can now be done at any time, because the underlying implementation uses MethodHandles and SwitchPoints.

The hidden system property that can enable it by default still exists, but it has been renamed and moved.
Merge pull request #3383 from chrisvest/2.2-new-pagecachemonitor

Refactor the PageCacheMonitor interface to be able to probe much deeper.
Expose a way to ask a lucene index to sample index selectivity

o Scan a sample of index entries and compute uniqueSampledValues / sampleSize
Add a new CountsKey for index entry counts
Maintain index update counts from index proxies

o OnlineIndexProxy will ensure that the index counts are updated
o Fix bug when generate LazyIndexUpdates to never construct 'overlapping updates'
o Cleanup code in LazyIndexUpdates
o Make sure we update index update counts during population
Add minor version to CountsStore such that we can flush safely on index flip (outside of log rotation)

o Ensure we compare CountsKeys correctly by type
o Fix reading of index keys
o Add flushIndexCounts() to IndexStoreView
Remove index counts when dropping an index
Make index stats part of SchemaRead and add indexNumberOfEntries

Rationale: For planning to be sane, indices shouldn't appear or disappear out of nowhere, hence we need to be tied to the schema read lock
Add IndexStatisticsTest
Pass on transaction id to index updaters
Pull out and refactor IndexCountVisitor from CountingIndexUpdater
Only apply index count updates for unprocessed transactions during recovery

o Rename updateXYZ to incrementXYZ CountsStore related methods
o Push transactionId down to IndexUpdaters (and couple them to process only updates for a single transaction)
Replace use of LongRegister with DoubleLongRegister in counts store
Add new type of IndexKey for storing sampling results
Refactored counts package

o CountsAcceptor is now called CountsAccessor and also has read methods
o CountsState => CountsRecordState
o IndexKey is now split into Index{Size|Sample}Key
o CountsTrackerState now uses explicit calls per key type
Support index samples in counts tracker
Implement index sampling
- initial index counts are computed during population
- index sampling is scheduled every 10 seconds
Hook index selectivity into Cypher
Make max unique elements per sampling configurable