Parse flag thresholds and provide them as double values in BridgeNetworkStatus and RelayNetworkStatusVote

Sample data:

https://collector.torproject.org/recent/bridge-descriptors/statuses/20151116-150531-4A0CCD2DDC7995083D73F5D667100C8A5831F16D

flag-thresholds stable-uptime=3768284 stable-mtbf=2721929 fast-speed=55000 guard-wfu=98.000% guard-tk=691200 guard-bw-inc-exits=364000 guard-bw-exc-exits=366000 enough-mtbf=1 ignoring-advertised-bws=0

https://collector.torproject.org/recent/relay-descriptors/votes/2015-11-16-15-00-00-vote-EFCBE720AB3A82B99F9E953CD5BF50F7EEFC7B97-0B4498C736621BDD0867DE4D56D2D08B9C7B7215

flag-thresholds stable-uptime=1291379 stable-mtbf=460242 fast-speed=102000 guard-wfu=98.000% guard-tk=691200 guard-bw-inc-exits=4670000 guard-bw-exc-exits=3820000 enough-mtbf=1 ignoring-advertised-bws=1