fix: Avoid source-map module during signal-exit handler

[coreyfarrell]

Prior to this change child processes would call upon the source-map
module to remap coverage during the signal-exit handler. This was a
blocker for updating source-map to 0.7.x which uses an async
constructor method.

Now we simply save the source-map with the raw coverage data for each
file. This allows the main nyc process to perform source-map remapping.
The benefit is that the main nyc process can use async code.

[coveralls]

Coverage increased (+0.002%) to 99.14% when pulling 7aacf58 on coreyfarrell:less-signal-exit into 22a5ea0 on istanbuljs:master.