Make sdcard daemon multi-threaded.

The essential idea here is that a handler thread only needs to
hold a lock on the global node table while it is manipulating
nodes.  The actual I/O operation is then performed without
holding any locks.

By default, we use 2 threads but this can be configured on the
command-line.  Work is sheduled somewhat arbitrarily by the
handler threads.  Whichever thread happens to read() the next
request first wins the right process it.  This policy is very
simple but potentially wastes threads when there isn't much
work to be done.  We can always improve this later if needed.

Change-Id: Id27a27c2c9b40d4f8e35a6bef9dd84f0dfacf337