time_window_compaction_strategy: reject invalid window_sizes

Scylla mistakenly allows an user to configure an invalid TWCS window_size <= 0, which effectively breaks the notion of compaction windows.
Interestingly enough, a <= 0 window size should be considered an undefined behavior as either we would create a new window every 0 duration (?) or the table would behave as STCS, the reader is encouraged to figure out which one of these is true. :-)

Cassandra, on the other hand, will properly throw a ConfigurationException when receiving such invalid window sizes and we now match the behavior to the same as Cassandra's.

Refs: #2336

compaction/time_window_compaction_strategy.cc

@@ -43,6 +43,10 @@ time_window_compaction_strategy_options::time_window_compaction_strategy_options
         }
     }
 
+    if (window_size <= 0) {
+        throw exceptions::configuration_exception(fmt::format("{} must be greater than 1 for compaction_window_size", window_size));
+    }
+
     sstable_window_size = window_size * window_unit;
 
     it = options.find(EXPIRED_SSTABLE_CHECK_FREQUENCY_SECONDS_KEY);