We enable FUSE_CAP_ASYNC_READ per default, which means that
the kernel can (and does) submit multiple concurrent out-of-order
read requests to service userspace reads and kernel readahead.

For some backing storages, like Amazon Cloud Drive, out-of-order
reads are expensive.

gocryptfs has implemented a delay-based workaround with its
`-serialize_reads` flag for this case
(see https://github.com/rfjakob/gocryptfs/issues/92 for details).

Not enabling FUSE_CAP_ASYNC_READ makes the kernel do this for us,
as verified by adding debug output to gocryptfs, so expose it as
a mount flag in MountOptions.

Fixes: https://github.com/hanwen/go-fuse/issues/140
Graphs-at: https://github.com/hanwen/go-fuse/issues/395
Related: https://github.com/rfjakob/gocryptfs/issues/92
Change-Id: I10f947d71e1453989c4a9b66fbb0407f7163994f