MINOR: memprof: prepare to consider exec_ctx in reporting

MINOR: memprof: prepare to consider exec_ctx in reporting

This now allows to report the same function in multiple bins based on the
th_ctx's exec_ctx discriminant. It's also worth noting that the context is
not atomically committed, but this shouldn't be a problem since a single
entry can get it. In the worst case, a second thread trying to create the
same context in parallel would create a different bin just for this call,
which is harmless. The same situation already exists with the caller
pointer.