In WT-3555 we tried to avoid making a copy of table URIs every time a cursor is opened.

That change turned out to be unsafe, because we open a "file:" cursor in that case and it does not reference the "table:" dhandle. As a consequence, the table dhandle can be closed by a sweep while there is a file cursor open with a pointer to its URI. WT-3730 reverted the original change.

Since there was a measurable performance improvement from this change, and we don't want table dhandles to be discarded when they are in use by cursors, we should investigate reinstating the change in WT-3555 but further having the cursor keep a reference to the "table:" dhandle and only releasing it on close.