Add quick and dirty load performance instrumentation

Instruments the client-side XHRs to add Chrome devtools timeline events and
console timing for loads.  Also adds a log message to the python code to
track timing and sizes of loads.
Read search optimization: request only the fields we use

This omits, for example, the unused alignedBases and tags field of reads which
can be quite large.  In one example of navigating a 1000 genomes readset at
the 'reads' scale, this reduced data transfer volume for a typical navigation
from 400kB to 150kB, and client load time from about 2s to 1.5s.