Make coalescing first set of results optional, disable by default.

Add an option to the prox plugin to make coalescing the first set of
declarative (non-manual) query results optional. Coalescing them
sounds nice because it gets rid of any intermediate steps that need to
be reported and dumps all the data to the client at once. However, it
means we need to wait for the initial query evaluation to finish,
which could take a bit when the tree is large. Disable coalescing by
default.
Update for ZernikeDescriptor bug fix in libprox.
Fix bug in tracking available SST segments.

Add a bunch of tests for SST::ReceivedSegmentList, including adding in
various orders, merging in different directions, and repeatedly
inserting parts of segments. This tracked down a bug in the
implementation of ReceivedSegmentList where the wrong segment was
being modified (it should have been next_it, not it).
Add a few more simple SST segment tests.

Check that merging segments and repeated insertion of partial
segments that don't touch the boundaries of the existing segment
(i.e. repeat of a middle packet where 3 packets are already available)
work properly.