Improve performance of the state.json endpoint for large clusters.

The master's state.json endpoint consistently takes a long time to compute the JSON result, for large clusters:

$ time curl -s -o /dev/null localhost:5050/master/state.json
Mon Jan 26 22:38:50 UTC 2015

real	0m13.174s
user	0m0.003s
sys	0m0.022s

This can cause the master to get backlogged if there are many state.json requests in flight.

Looking at perf data, it seems most of the time is spent doing memory allocation / de-allocation. This ticket will try to capture any low hanging fruit to speed this up. Possibly we can leverage moves if they are not already being used by the compiler.