brick_server client API enhancement - get/4 with byte range option

[tatsuya6502]

Add new operation flag range to get/4 operation:

• {range, Start::non_negative_integer(), Length::integer()}

Start is a zero-based offset into a value of the key and Length is the length of that part. (Will decide more details later.)

One of the major benefits of issue #17 (Redesign disk storage and checkpoint/scavenger processes) is that now value blob can be read without deserializing a log hunk (no need to apply binary_to_term/1 to it), so this feature can be easily implemented and will have no performance overhead.