cql3::util::maybe_quote: further optimize quote doubling
cql3::util::maybe_quote: further optimize quote doubling

Avoid string copies when doubling quotes in the string
by counting them when scanning the input string and
reserving the required space when making the result std::string.

This showed a performance improvement of ~1.8% when
running the maybe_quote unit test in tight loop
(w/ the shorter strings only)

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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