Report bytes written in query results statistics

[jkylling]

It would be useful for clients of the HTTP REST API which are iterating over the nextUris of some write operation to know the approximate number of bytes written so far, in addition to the number of bytes processed or CPU time used. This can for instance be used to cancel queries writing too much data early.

The physicalWrittenDataSize is already part of the infoUri, but this URL is more expensive.

The StatementStats created here could be extended to include queryStats.getPhysicalWrittenDataSize().toBytes().

[electrum]

This seems reasonable. Feel free to submit a PR.