snapuserd: Add diagnostics for debugging races.

Adds calls to pthread_setname_np for each thread.

Clarify error messages from io_uring calls that return -errno.

Add log messages for some failure paths that didn't have any.

Add an ostream overload for MERGE_IO_TRANSITION, and add an INVALID
state for initialization.

Bug: 288273605
Test: builds
Change-Id: Ic0681cbf0017af67bcf52b98db184a9b48752faf