cql3: Specialize partition-range computation for EQ
cql3: Specialize partition-range computation for EQ

Save a couple of allocations per request by treating all-EQ cases
specially during the computation of partition ranges.

Signed-off-by: Dejan Mircevski <1e9694aae17a51d4875d9bf4a0d0e20ef37c1b85@scylladb.com>

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