wip: refactor abstract mapreduce

[emilygilberts]

did some ES6 refactoring:

• converted functions updatePurgeSeq, saveKeyValues, createTask, getRecentPurges to async/await to replace promise chains

• refactored postProcessAttatchments to accept the resolved result (res) directly, wich enables to remove the .then()` on the call

• removed unneccessary promise wrappers around calls of:

• postProcessAttatchments (is now invoked directly with the resolved result - instead of with .then() chaining)

• queryPromised (the async function already returns a promise)

• replaced the use of fin (final promise factory function) with try/finally block

• removed the last var

note:
currently repeadedly writing out the defaultsto functionality, will take care of it next :)