Don't clean up memory in AdminRequestHandler::abortRequest

Summary: We clean up teardownRequest also, which is going to run right
after abort, so I think we end up destroying g_context twice.

Reviewed By: @markw65

Differential Revision: D1663440

Signature: t1:1663440:1415284204:e79ace99f3d4b1fe15b7531542bb9f88661274e8
Use a global transliterator with locking instead of TLS

Summary: icu::Transliterator manages an internal cache of various things like
Unicode character sets.  Having one per thread wastes 10s to 100s MBs, and it's
not a very performance-critical structure (at least for www).

Reviewed By: @billf

Differential Revision: D1658186