Allow retry from failure other than just at commit.
Need whole new transaction, rather than reuse.
Also work for periodic commit mode.
Reserve a couple more codes.
Confusion between # of retries and # of attempts.
There was already a named class doing what this anonymous class was doing.
Clean out all inactive AIS generations, not just those older than the oldest.
merge MMcM: Get rid backpointer from IndexScan operator to context that compiled it

Merge pull request #1209 from MMcM/sql-layer/sql-column-selector-context-reference
https://github.com/FoundationDB/sql-layer/pull/1209
Tweak the logging a little to include the particular error.
merge MMcM: Redo row reader retry logic

Merge pull request #1205 from MMcM/sql-layer/row-reader-refine-retry
https://github.com/FoundationDB/sql-layer/pull/1205
merge MMcM: Clean out all inactive AIS generations, not just those older than the oldest

Merge pull request #1210 from MMcM/sql-layer/sql-ais-active-generations
https://github.com/FoundationDB/sql-layer/pull/1210