added size checking for bson builders in locking diagnostics code #524

With this, we'll stop early if we would get close to the 16MB size limit,
and say there were too many results.  In the future we can return a cursor
from the command, to stream results back and get around the size limit.
added shell wrappers for lock diagnostics #437
unified names of txnids returned in bsons #437
updated NEWS for 1.2.1