Race condition inside merge lines

Depending on how fast you merge lines, the update doesn't always fire quite right and isntead of merging lines, one line takes on the properties of both while the other still exists, which makes it appear as if there are overlapping lines. This also occurred when just doing one merge, so I think the race condition is internal to the mergeLine function since it fires ~4 calls to the mongo to perform.

Start by tracing the mergeLine function and the roughly four MongoDB calls involved in a single merge. Reproduce the failure with one merge and rapid successive merges; done means the lines merge consistently without overlapping records or leaving both lines present.

Written by the indexing model from the issue text.