cell_comparator: Better fix (i.e. potentially correct) for compound/clustered desc.
cell_comparator: Better fix (i.e. potentially correct) for compound/clustered desc.

As Tomek pointed out, previous code, regardless of version mismatch, of generating
comparator description string was not correct (as in: in sync with origin).
This modifies it to look at
1.) Actual clustring size
2.) Compound-ness
3.) Dense-ness

to determine whether we should generate a compound desc, and whether it
should contain a trailing utf8-desc type.

v2: Simplify non-dense base column addition and ensure it handles
    thrift non-utf8 (as per comments from tomek)
Message-Id: <fca2e535a228b505c286c001eda80981fd6e71d4@scylladb.com>

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