ENG-4384: Add a CLI arg to disable adhoc writes in txnId-selfcheck2.
ENG-4384: Add a naive rate limiter to txnId-selfcheck2.
ENG-3154,ENG-3153,ENG-3163,ENG-4357: Support and prep code for schema change.
Also some typo fixes.
No functionality changes.
More smallish changes from the schema branch to the prep branch.
Merge remote-tracking branch 'origin/master' into ENG-3153-schemaprep
Fix table generation bug and add a better comment to a TableHelper method.
Cleanup of the schema-prep branch per pmartel feedback.
Merge remote-tracking branch 'origin/master' into ENG-3153-schemaprep
ENG-4384: Set spHandle on CompleteTransactionMessage.

CompleteTransactionMessages truncates the FragmentTaskMessages in the repair
log. On SPI repair, if the complete message doesn't have the latest spHandle,
the new master may generate an spHandle that's already been used before. Set the
spHandle for each complete message so that they can advance the max seen
spHandle on repair.
Merge pull request #386 from VoltDB/ENG-3153-schemaprep

Eng 3153 schemaprep
ENG-4306 Make VoltTableUtil.CSV_NULL public for use by client programmers.
ENG-4384: Move txnId-selfcheck2 permit reset code to the rate limiter per John's
review.
Merge pull request #387 from VoltDB/ENG-4384

ENG-4384: Modifications to txnId-selfcheck2 for DR tests
ENG-4402: Fix deadlock with backpressure callback in Distributor.
Revert "ENG-4402: Fix deadlock with backpressure callback in Distributor."

This reverts commit f07ae08c59bf37b574a2889adc0af9c915bd82e2.
ENG-4402: Fix deadlock with backpressure callback in Distributor.
ENG-4382: Add unit tests for old-style @SnapshotSave argument parsing in SnapshotDaemon.
ENG-4382: Add some quick unit tests for JSON param to @SnapshotSave
ENG-4382: Extract initial legacy and json param validation
ENG-4382: Create SnapshotInitiationInfo, start threading it around, not yet used