Inserting index metadata within index backstores upon creation.

Specifically:
- Adding new class lf.index.IndexMetadataRow.
- Upon creation of index backing stores inserting a IndexMetadataRow
with a fixed ID in the backing store (both for IndexedDB and MemoryDB).
This will allow to later deserialize different types of indices (type
is stored in the metadata).
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81466734
Make B-Tree handle multiple values per key.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81468706