ovs-thread: Add new support for thread-specific data.

ovs-thread: Add new support for thread-specific data.

A couple of times I've wanted to create a dynamic data structure that has
thread-specific data, but I've not been able to do that because
PTHREAD_KEYS_MAX is so low (POSIX says at least 128, glibc is only a little
bigger at 1024).  This commit introduces a new form of thread-specific data
that supports a large number of items.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>