thrift: Implement multiget_count verb
thrift: Implement multiget_count verb

This patch implements the multiget_count verb in a similar fashion as
multiget_slice, but using an accumulator that counts the returned
columns instead of create thrift ColumnOrSuperColumn objects.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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