[leo_backend_db][eleveldb] Iterator-handler can be leaked

[mocchira]

While tackling #710 , I noticed there is a bug that causes an iterator handle to leak.
Once we call eleveldb:iterator, we must call eleveldb:iterator_close/1 (https://github.com/basho/eleveldb/blob/2.0.34/src/eleveldb.erl#L246-L250) otherwise the iterator handle is leaked.

TOBE

Should be implemented with try after manner like the below.

• https://github.com/basho/eleveldb/blob/2.0.34/src/eleveldb.erl#L260-L271
• https://github.com/basho/eleveldb/blob/2.0.34/src/eleveldb.erl#L379-L394

[mocchira]

Note: The problem exists only at leo_backend_db_eleveldb.
(leo_backend_db_bitcask has been properly implemented)