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

• rewrote the defaultsTo function that sets a default value if a promise throws a 404 error - which used to fit neatly into the.catch(defaultsTo(x).then(..) syntax, but couldn't be used anymore after the rewrite. I added an optional property parameter to the function, as we sometimes want to return the awaited result directly (ex. lastSeqDoc in saveKeyValues) and sometimes a property of it (ex. purqueSeq._rev in updatePurgeSeq). This part was most confusing for me - so I'm happy for any thoughts on it!