optimize geojson writer

Conversion from double value to char representation
is killing performance. This change replaces ostreams
with string appends to speed it up.

Change-Id: Ice0e8694deeb8898340dd7c989689aa4511c7fb4
more optimization of geojson writer

Move from strings to sprintfs to be faster still.
Requires manual buffer management of course.

From here we have to move to bson and then to
more compact representations.

Change-Id: I043fe5e4b3308449ac30092fa7128c7475ff47d8
Merge topic 'optimize-geojson' into master

0e46e310 more optimization of geojson writer
bd3c09e3 optimize geojson writer