A `writer_node` contains a chain of multiple BTI nodes.
`writer_node::_node_size` is the size (in bytes) of the
entire chain.
But the parent of the `writer_node` wants to know the offset
to the rootmost node in the chain. This can be deduced from the
`writer_node::_transition_length` and the `writer_node::_node_size`.
But there's an error in the logic for that, for the special case
when there are two nodes in the chain.
The rootmost node will be SINGLE_NOPAYLOAD_12 if and only if
*the leafmost node* is smaller than 16 bytes, which is
true if and only if `_node_size` is smaller than 19 bytes.
But the current logic compares `_node_size` against 16.
That's incorrect. This patch fixes that.
There was a test for this branch, but it was not good enough.
It only tested payloads of size 1 and 20, but the problem
is only revealed by payloads of size 13-14.
The test has been extended to cover all sizes between 1 and 20.
Adds tests which check that nodes serialized by `bti_node_sink`
are readable by `bti_node_reader` with the right result.
(Note: there are no tests which check compatibility of the encoded nodes
with Cassandra or with handwritten hexdumps. There are only tests
for mutual compatibility between Scylla's writers and readers.
This can be considered a gap in testing.)
