Allow multiple calls to syncFileIndex to batch up

[tvoliter]

See todo in FileIndexManager.syncFileIndex.

Right now this function does not permit recursive calls which means the client code must write code async code that doesn't queue a request while the index is being updated.

[jlondon]

Closing for now but we'll deal with this when we implement our first async story.

[peterflynn]

I have a fix for this. It makes certain usage of QuickOpen APIs difficult/impossible, and every once in a while I'll see crashes in vanilla in-product Quick Open usage as a result of this (typically if there's a lot of disk churn causing the index rebuild to run slow).

[peterflynn]

Adding FBNC & sprint tag now that this is actually fixed.