thrift: Implement get_multi_slice verb
thrift: Implement get_multi_slice verb

The get_multi_slice verb is used to perform multiple slices on a
single row key in one operation. It takes a set of column_slices,
which we normalize to not contain any overlapping ranges.

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

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