Cassandra SE: small features

This is a task for small, 'apparent' features needed in Cassandra SE.

• Add @@cassandra_write_consistency_level and @@cassandra_read_consistency_level which will allow to specify consistency levels for reads and writes.

• Add a global cassandra_thrift_host/port setting. Hosts/IP addresses change across EC2 runs, which means one has to re-create table definitions.

• Check how "ALTER TABLE tbl option=value" command works. With SphinxSE, it will create another table, then try to copy all of the data from the original table. This will still look OK for the user, because "select * from sphinx_table" never returns any rows.