Previously, alternator server was not directly sharded - and instead
kept a helper http server control class, which stored sharded http
server inside. That design is confusing and makes it hard to expand
alternator server with new sharded attributes, so from now on
the alternator server is itself sharded<>.
Tests: alternator-test(local, smp==1&smp==4)
Fixes#5913
Message-Id: <b50e0e29610c0dfea61f3a1571f8ca3640356782.1582788575.git.sarna@scylladb.com>