Adding a thread-safe wrapper around the ADC

[mtlynch]

Multiple threads share access to the ADC instance, so we need a way to make it thread-safe.

[coveralls]

Coverage increased (+0.1%) to 82.418% when pulling fe031030f389cf30e634b505f566bf2ccd930fb7 on mtlynch:thread-safe-adc into f4e9d21c8fa3f3ecd984be5294f335f424d2a1a8 on JeetShetty:master.