In the next patch decoding a pickle will depend on whether current
protocol is <= 2 or >= 3. For this let's teach PROTO opcode handler to
recall last seen protocol version.

For testing - prepare tests infrastructure for cases where protocol
version affects decoding semantic: if a test pickle in tests table comes
with \x80\xff prefix, on decode tests this prefix will be changed to
concrete

	\x80 ver

with all versions protperly iterated as specified in TestPickle.protov.

We will also need this functionality in the next patch.