contentenc: parallelize encryption for 128kiB writes
contentenc: parallelize encryption for 128kiB writes

128kiB = 32 x 4kiB pages is the maximum we get from the kernel. Splitting
up smaller writes is probably not worth it.

Parallelism is limited to two for now.

rfjakob/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs