Add leveldb to the thirdparty installer

Signed-off-by: jkaiser <juer.kaiser@gmail.com>
Add check for leveldb to SCons script

Signed-off-by: jkaiser <juer.kaiser@gmail.com>
Add new capability PUT_IF_ABSENT

Only index structures with this capability ensure that
the PutIfAbsent method is supported. Other index implementations
should return an error if PutIfAbsent is called.

Signed-off-by: jkaiser <juer.kaiser@gmail.com>
Add basic Leveldb index.

Leveldb is a log-structured merge tree implementation using in different
database products, e.g. in Riak.

Signed-off-by: jkaiser <juer.kaiser@gmail.com>
Add leveldb-based configurations to central core unit tests

Signed-off-by: jkaiser <juer.kaiser@gmail.com>
Add leveldb-based configurations to central dedupv1d unit tests

Signed-off-by: jkaiser <juer.kaiser@gmail.com>