Support range sortKey

[zhoney]

Description

SortKeys of EdgeLabel is implemented like a secondary index now. We can only provide actual sortKey value to query if wanting to query by sortKey. But, querying by sortKey range is needed In some conditions. For example, If Call edges connect two PhoneNo vertices and use timestamp as Call's sortKey, querying by timestamp range seems more reasonable rether than querying by a timestamp.

[javeme]

we can encode int or long to base64 string (LongEncoding)