Find UniqueId suggestions asynchronously.

This is the first patch in a series to make all the UniqueId code
asynchronous / non-blocking.

Change-Id: I04ee352d3e457f4e546217d6d9167beb760b24a6
Add a helper method for asynchronous internal errors.

Change-Id: I60c240a3d386902ecea9edd06f56d3d7d439b094
Serve /suggest asynchronously.

Change-Id: I77b315f0f2ec39778b350abfbfe09d81187f8bd0
Deprecate UniqueIdInterface.

This interface has never been useful and will eventually be removed.

Change-Id: Ie63411f17486074a4dd96829af09d172363cc1f2
Make UniqueId lookups asynchronous.

Change-Id: I13465798cd69cbb8d43ddb52b8c55ad2cf28fcf0
Factor out a bit of UniqueId code.

Change-Id: I1b7dd4aaa689ad36f5b926ae14789ce2a8a2ec10
Coding style fixes.

Rename some variables to be consistent with the rest of the code base.
No actual code change.

Change-Id: I5486bc817022027b6b7e5f485a93fdc85837da27