Replace Traversable with Visitable, simplify Visitor
merge nathanlws: Replace Traversable with Visitable, simplify Visitor

Merge pull request #1014 from nathanlws/sql-layer/traversable-to-visitable
https://github.com/FoundationDB/sql-layer/pull/1014
Make sure TransactionService.run is called in a clean transaction, creating a separate session if that's what it takes.
merge MMcM: Make sure TransactionService.run is called in a clean transaction

Merge pull request #1015 from MMcM/sql-layer/persistit-run-separate-txn
https://github.com/FoundationDB/sql-layer/pull/1015
Remove standin storage descriptions from AISBuilder.
merge MMcM: Remove standin storage descriptions from AISBuilder.

Merge pull request #1016 from MMcM/sql-layer/remove-standin-storage
https://github.com/FoundationDB/sql-layer/pull/1016
Validator always does index changes
Minor cleanup, reduce duplication
merge nathanlws: Remove unused strict index changes from TableChangeValidator

Merge pull request #1017 from nathanlws/sql-layer/always-auto-index-changes
https://github.com/FoundationDB/sql-layer/pull/1017
Reserve slot for for other adapter error.
merge MMcM: Reserve slot for for other adapter error.

Merge pull request #1018 from MMcM/sql-layer/error-code-53004
https://github.com/FoundationDB/sql-layer/pull/1018
Restore exception for unsupported ALTERs
merge nathanlws: Restore exception for unsupported ALTERs

Merge pull request #1019 from nathanlws/sql-layer/no-alter-add-index
https://github.com/FoundationDB/sql-layer/pull/1019