serialization_visitors: Adding vector_position struct
serialization_visitors: Adding vector_position struct

While serialization vector it is sometimes required to rollback some of
the serialized elements.

vector_position is the equivalent to the bytes_ostream position struct.
It holds information about the current position in a serialized vector,
the position in the bufffer and the current number of elements
serialized.

It will allow to rollback to the current point.

Signed-off-by: Amnon Heiman <ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@scylladb.com>
Message-Id: <1456041750-1505-2-git-send-email-ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@scylladb.com>

raphaelsc/scylla,avikivity/scylla,avikivity/scylla,raphaelsc/scylla,kjniemi/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,kjniemi/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla