SERVER-2212 Misc. comments, formatting
SERVER-2212 Add CurOp public method elapsedMicros()
SERVER-2212 Ability for CurOp objects to be "timer-interruptible"

- New public methods introduced to CurOp for requesting op
  interruption after a given time period.
- KillCurrentOp objects now additionally tasked with interrupting
  CurOp objects that have exceeded their time limit, during the
  "interrupt check".
SERVER-2212 Logic to apply time limits to getmore CurOp objects

If a CurOp object generating a cursor has a time limit imposed,
remaining time is "rolled over" to the ClientCursor object, and
applied to later getmore ops.  Affects getmore ops on cursors
associated with both normal queries and cursor-enabled aggregation
commands.
SERVER-2212 Add mongod query option $maxTimeMS, shell helper

$maxTimeMS can be added as a top-level field to any query or
command ($cmd.sys.* ops not included).  Requests op interruption
after the user-given time limit.