allow multiple on demand requests to be queued up

[ssalinas]

/cc @jhaber @jonathanwgoodwin

[ssalinas]

@wsorenson @tpetr traced back through the different PendingTypes we could possibly get in combination with the ONEOFF type. Those are: RETRY, DECOMISSIONED_SLAVE_OR_RACK, UPDATED_REQUEST, or TASK_DONE. No others are currently created for an on demand request.

RETRY -> create another identical oneoff, there is no conflict here as both kinds just schedule a single task with the relevant options
DECOMISSIONED_SLAVE_OR_RACK - > no problem there, no new tasks will be scheduled for a oneoff
TASK_DONE -> Similar, we do not schedule new tasks for a oneoff on task done
UPDATED_REQUEST -> This was the only possible relevant one (for values like killOldNonLongRunningTasksAfterMillis which I don't believe we wired up for on demand anyway). However, when we encounter this pending type the update to request data is already finished and we do not schedule any new oneoff tasks for a UPDATED_REQUEST pending type

[ssalinas]

quick addition, misread an if statement, we in fact do not get a UPDATED_REQUEST pending type for on demand requests

[jhaber]

There are no words to express my excitement 👯

[ssalinas]

This has been performing well with no issues, going to merge