Make circ_times static and add accessor functions.

Change the global circ_times to a static variable and use
accessor functions throughout the code, instead of
accessing it directly.
Rename get_circuit_nbuild_* to end with _ms
Separate mutable/const accessors for circuit_build_times

(These have proved invaluable for other global accessors.)
changes file for 6153
Documentation for new cbt accessors
Hide the contents of the circuit_build_times structure.

There were only two functions outside of circuitstats that actually
wanted to know what was inside this.  Making the structure itself
hidden should help isolation and prevent us from spaghettifying the
thing more.
Merge branch 'bug6153_squashed'