threads: fine grained locking for Thread

threads: fine grained locking for Thread

Until now many accesses to the Thread structure required taking a global
lock, leading to performance issues. In practice this only happened in
offline mode.

This patch adds a finer grained locking scheme. It assumes that the
Thread object itself cannot disappear, and adds a spinlock to protect
updates to the structure.

Additionally, the `pktts` field is made an atomic, so that it can be
read w/o taking the spinlock. Updates to it are still done under lock.