removed the 'vlen' argument from most of the interfaces

at least from those where it's irrelevant to keep track of the value size
since the value itself is never touched by the library and if the caller
needs to keep track of it can simply make use of some wrapper structure where
both the data pointer and the length can be stored

the only place where keeping track of the length really matters is where,
for concurrency purposes, copy functionalities are provided hence the library
needs to know the size of the value to be able of making a safe copy
when its internal lock are retained (e.g. hasthable.[ch])