move loadMerkleTree to v1/state.go

[luispdm]

• loadMerkleTree renamed LoadMerkleTrees and moved to State interface
• SetTreesForContract, IsTreeSetForContract, and CreateTreesForContract removed from State interface
• state_test.go moved to state_wb_test.go inside package v1, to easily set merkle tree mocks for the state transaction
• state_test.go now belongs to v1_test and it tests LoadMerkleTrees