Add a logToAccessLog() virtual to RequestHandler

Reviewed By: @markw65

Differential Revision: D1653311

Signature: t1:1653311:1414801532:207d3184f139fc35adf03bb69716baf5c33ee377
Mechanism for enabling request-local heap profiling

Summary: Adds an admin server command to trigger request-local profiling on the
next request.  This turns off the slab allocator and triggers a
thread-local jemalloc heap profile dump at request shutdown.

Reviewed By: @jasone

Differential Revision: D1491984