main: extract scylla specific code into scylla_main()
main: extract scylla specific code into scylla_main()

main() now contains only generic setup and teardown code and it
delegates to scylla_main().
In the next patches we want to wire in tool executables into the scylla
one. This will require selecting the main to run at runtime.
scylla_main() will be just one of those (the default).

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