[IOTDB-1597] Finish DIFF encoding algorithm which is reserved in enum before

[5Reeson]

In the enum class: TSEncoding, there's DIFF encoding, so when user want to choose a type of encoding algorithm(or input the wrong encoding type name), DIFF encoding is in the output hint and is one of IoTDB's recommending encoding type,
but in IoTDB, DIFF encoding has not been added before, which I think may cause some problems.
I think it's necessary to finish it.

Both DIFF and TS_2DIFF encoding are based on delta encoding algorithm,
so I finish DIFF using similar interfaces and structures like TS_2DIFF to make it easier to understand.

[jixuan1989]

please apply this patch to solve the conflicts.
git apply solve_conflict.txt
solve_conflict.txt

[jixuan1989]

1. there are too many duplications between DIFFEncoder and DeltaBinaryEncoder. Better to inherit DeltaBinaryEncoder, rather than copy codes.

2. What is the benefit to add such a method? Trying to give some experimental results to show when should we use it...

[5Reeson]

1. there are too many duplications between DIFFEncoder and DeltaBinaryEncoder. Better to inherit DeltaBinaryEncoder, rather than copy codes.
2. What is the benefit to add such a method? Trying to give some experimental results to show when should we use it...

I have made some experiment abouot it(https://cwiki.apache.org/confluence/display/IOTDB/DIFF+encoding%27s+perfomance).
As I said in email-list, this method's performance is not better than TS_2DIFF (included in 'DeltaBinaryEncoder'), only when the data set is in best circumstances, the new method can save very little room (about 4 bytes, in head)

I add it beacuse DIFF is a reserved method, it's one of the recommending method when user want to choose a encoding method in IoTDB, but it's not finished before. It's in the system prompt, but user can't chooose it

[JackieTien97]

Hi, please resolve the conflicts

[HTHou]

Looks better now.

[coveralls]

Coverage increased (+0.01%) to 67.374% when pulling d9881b2 on 5Reasons:addDiff into fb18357 on apache:master.