Enhanced Stats Collection and Logging on Sync Gateway

This feature is a precursor to mobile manageability functionality planned for the Mercury release

• Enhance the set of stats reported through _expvars so it is more meaningful to the end user.
• Log relevant stats in a separate stats specific log file for aid in debugging by support teams

PRD: https://docs.google.com/document/d/1N8OGl1ow10KvmoY-KgPuBLdKM-yPsbFcMjsK1ZFnN-w/edit#

Priority of feature : https://docs.google.com/spreadsheets/d/1K5EIWoiZifJt1ABygmj_8ppT-6_xS7ojhjxLEOgpHsU/edit#gid=0

Stats list: https://docs.google.com/spreadsheets/d/1aZjCIwwZYrUSaQCK63QUg7_jrbo0jJcsxPoXG2Q4s0o

Stats design doc: https://docs.google.com/document/d/1lBTzcnXSFo_C6cEiTIbUaeYaNPk_i4eMkz6Wumwku3w

Sample expvar.json attached.  The new stats are under the syncgateway top level key.