Add thread type and pagelets started functions

Summary: Caveat: I have close to no idea what I'm doing here. Don't
hesitate to tell me if everything is wrong, ugly, or will explode.

Add some very basic instrumentation around pagelet requests.
Currently we don't have any idea how many pagelets a request starts or
(AFAICT) a way to tell whether a request is a pagelet, besides looking
for PHP implementation details like parts of the request query string.

1) ##hphp_thread_type()## -- what kind of thread is this request running
in? Returns an int from ##enum class ThreadType## or -1 if we can't work
it out. There is an existing function (##getThreadTypeName()##,
https://fburl.com/65216334) but this returns strings with spaces, which
are a pain for dyno logs.

2) ##pagelet_server_tasks_started()## -- return the count of pagelets
started in this request. I didn't do this in PHP as there are already
two places in which we start pagelets (GraphBatchAsyncWorkItem and
PageletServerFuture) and two places in HHVM to start pagelets
(pagelet_server_task_start and fb_gen_pagelet). Instrumenting the server
is easier than making sure all the callsites in PHP track this.

I added both of these as functions and not access log parameters so we
can easily add them to xenon and other profilers.

Reviewed By: @bertmaher

Differential Revision: D1667021