Add IndexedInts.debugToString() and AbstractIndex.toString(); Add Sequence.toList() and limit()

[leventov]

• Added IndexedInts.debugToString(). Configure your IDE to render IndexedInts impls using this method during debug.
• Added toString() to QueryableIndex and IncrementalIndex via AbstractIndex

Unrelated:

• Added Sequence.toList() and limit(), simplify code.

[pjain1]

@leventov This PR breaks indexing tasks, toString() method in the AbstractIndex causes indexing tasks to log all the events present in the index every time incremental persists happens - https://github.com/druid-io/druid/blob/master/server/src/main/java/io/druid/segment/realtime/appenderator/AppenderatorImpl.java#L1156 ultimately causing them to fail with out of memory conditions.