refactor wildcard matching

refactor wildcard matching

refactor the wildcard matching code to make it a bit easier to
understand, in hopes that it will reduce the difficulty of converting
from RBTDB to QPDB later.

there are also some minor optimizations: previously, after stepping
backward to find the predecessor, we stepped back foward *from* the
predecessor to find the successor.  we now reset the rbtnode chain to
its original starting point before stepping forward; this eliminates
some unnecessary processing. and, if neither predecessor nor successor
is found, we return early rather than carrying on with an unnecessary
effort to match labels.