add pruning stage concept, but not enable yet (#2385)

cmd/integration/commands/stages.go

common/dbutils/bucket.go

eth/stagedsync/default_stages.go

eth/stagedsync/stage.go

eth/stagedsync/stage_blockhashes.go

eth/stagedsync/stage_bodies.go

eth/stagedsync/stage_bodies_snapshot.go

eth/stagedsync/stage_call_traces.go

eth/stagedsync/stage_execute.go

eth/stagedsync/stage_finish.go

eth/stagedsync/stage_hashstate.go

eth/stagedsync/stage_headers.go

eth/stagedsync/stage_headers_snapshot.go

eth/stagedsync/stage_indexes.go

eth/stagedsync/stage_interhashes.go

eth/stagedsync/stage_log_index.go

eth/stagedsync/stage_mining_create_block.go

eth/stagedsync/stage_mining_exec.go

eth/stagedsync/stage_mining_finish.go

eth/stagedsync/stage_senders.go