__wt_debug_addr() can use the standard block manager interface
add a comment why __wt_debug_offset() can't use the standard block manager API.
The mmap branch changes broke checkpoint memory management, verify was
leaking memory.

Quit pretending to do any lock checking in the block manager: we depend
on the session layer never (never!) opening the live system multiple
times, or creating, truncating, verifying or salvaging when the live
system is open, for that matter.
Fix an error message, we might someday have more than one checkpoint version.