Allow multiple 2ndary index on the same column

Currently, the code assumes that we'll only have one 2ndary index per column. This has been reasonable so far but stop being it with CASSANDRA-6382 (you might want to index multiple fields of the same UDT column) and CASSANDRA-7458 (you may want to have one "normal" index an multiple functional index for the same column). So we should consider removing that assumption in the code, which is mainly 2 places:

1. in the schema: each ColumnDefinition only has infos for one index. This part should probably be tackled in CASSANDRA-6717 so I'm marking this issue as a follow-up of CASSANDRA-6717.
2. in the 2ndary index API: this is the part I'm suggesting we fix in this issue.