support sstoragePisa.RemoveKVData

[cyl19970726]

The remove operation consists of two steps.

• First, replace the data corresponding to removeKvIdx selected by the operator with data of lastKvIdx
• Second, set the data space of the original lastKvIdx to 0

Test

• Tests are written in the contracts_test.go file.
• I prepare three shards to test.
  • Shard Info : MaxKvEntries = 10; KVSize = 4 1024; chunksPerKv = 1; ChunkSize = 4 1024
• Test case cover:
  • put
  • put test: the operation of storing data will fill up all shards
  • update
  • update test : the operation of updating data will update all data stored in each shard
  • remove
  • remove test:
    • case 1: removeKvIdx is equal to lastKvIdx
    • case 2: removeKvIdx and lastKvIdx are in the same Shard
    • case 3: removeKvIdx and lastKvIdx are in the different Shard