Use mgo.Collection.Bulk to prune docs

[axw]

Use bulk document removal, rather than
a single RemoveAll with a query of "_id $in [...]".
The latter has performance problems in some
configurations of MongoDB. See https://bugs.launchpad.net/juju/+bug/1635311.

Excerpt from the bug:

Today I did the test again on the Azure instance where the issue was reproduced, instead of my local system where I did it first. On there, Mongo does not like the method used at all. In the logs from Comment 6, there's a log message stating that ~9000 transactions need pruning. Using the method from Juju, removing 10000 docs took about 2 minutes in one test. I wrote some more optimised code which does the same thing ~instantly.

(Review request: http://reviews.vapour.ws/r/5955/)

[mjs]

This looks great for 2.0.

Before this gets used with 1.25 (MongoDB 2.4), it would be good to test the performance of mgo's Bulk fallback for older MongoDB versions.

[axw]

For posterity: I tested with mongo 2.4, and there is no performance penalty for the existing approach. Using Bulk isn't as fast there because 2.4 doesn't support bulk operations, and so mgo just turns it into a series of operations. Nevertheless, it only takes a few seconds to remove the 10000 documents, so we'll stick with just using Bulk across the board for simplicity.

[axw]

$$merge$$