types: Simplify and explain from_varint_to_integer
types: Simplify and explain from_varint_to_integer

This simplifies the implementation of from_varint_to_integer and
avoids using the fact that a static_cast from cpp_int to uint64_t
seems to just keep the low 64 bits.

The boost release notes
(https://www.boost.org/users/history/version_1_67_0.html) implies that
the conversion function should return the maximum value a uint64_t can
hold if the original value is too large.

The idea of using a & with ~0 is a suggestion from the boost release
notes.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla