Currently, when an index build is creating multiple indexes, the draining of each side writes table happens serially. This means that as one index drains, the other indexes can continue building up more side writes. There are two notable implications to this:

1. The index build can take an extremely long time to complete
2. The final drain under exclusive collection lock can have a non-trivial amount of work to do

In order to alleviate this, we may want to consider draining each side writes table in parallel with the others. This way the build can reach commit more quickly, plus there work to do at commit time under the exclusive collection lock will be minimized.