Check for quotaBytes on mutation.
Propagate error on old item deletion.

Also pass along the old item so we can de-dupe the changes stream.
Move some prep work outside of mutate() lock.
Helper vbucket.getUnexpired() function.
Simplified 2 locks (vbucket.Apply() and Mutate()) into 1 lock.
Compute deltaItemBytes at the partition level.

It's now more accurate accounting for for deletions and also reduces
the copy-&-paste code a little bit.
Cleanup after test.
TestBucketQuotaBytes().