This commit adds support for executing ALTER KS for keyspaces with
tablets and utilizes all the previous commits.
The ALTER KS is handled in alter_keyspace_statement, where a global
topology request in generated with data attached to system.topology
table. Then, once topology state machine is ready, it starts to handle
this global topology event, which results in producing mutations
required to change the schema of the keyspace, delete the
system.topology's global req, produce tablets mutations and additional
mutations for a table tracking the lifetime of the whole req. Tracking
the lifetime is necessary to not return the control to the user too
early, so the query processor only returns the response while the
mutations are sent.
fbd75c5c06