vinyl: truncate region after extracting key from tuple

tuple_extract_key() uses region to store the result. If the region is
not truncated, as it is currently the case, the memory consumption can
rocket sky high during a transaction execution. This is especially
critical in case of select() or pairs() over a secondary index.

To fix that, let's introduce vy_stmt_extract_key() wrapper, which would
store the result on malloc, and use it throughout the code. Note, this
doesn't add malloc() invocations, because malloc() has to be invoked by
vy_index_get() anyway - we just move it up.