Prevent conflict with newversion transaction

Prevent an error that was happening when attempting to clear the db
after the newversion (upgrade needed) transaction started but before
the db request was ready.
Prevent clobbering track file on initial upload

_playMyTrack expects the track file to be a file/blob, but when we
put the track in the db, it must be a string. Seperate these, and use
exportTrack to turn the file into a string.