LWT batches conditions can't span multiple tables.
This was detected in batch_statement::validate() called in ::prepare().
But ::cas_result_set_metadata() was built in the constructor,
causing a bitset assert/crash in a reported scenario.
This patch moves validate() to the constructor before building metadata.
Closes#6332
Tested with https://github.com/scylladb/scylla-dtest/pull/1465
Signed-off-by: Alejo Sanchez <alejo.sanchez@scylladb.com>
