Decompress off the main thread.

Summary: I didn't use more than one thread because it made no difference
to the timing (i.e. we are IO-bound)

Reviewed By: @elgenie

Differential Revision: D1693167

Signature: t1:1693167:1416455121:b20b7af6e813ab5d0f674c2b9432f764655223d2