redis: Simplify redis_worker_insert()

Instead of having a number of branches and formatting the Redis command
with redisCommand(), use redisCommandArgv(). This flattens the flow a
little and eliminates a needless string parsing & formatting step.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
redis: Turn self->command into a GString

Since we need the length of the command on every insert, turn it into a
GString, so we will always have the length readily available.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #25 from balabit/merge/f/redis-argv

Small Redis improvements