Rename HasBlock to NotifyNewBlocks, and make it accept multiple blocks

The goal here is twofold:
- allows for batched handling of multiple blocks at once
- act as a noisy signal for the breaking change that Bitswap is not adding blocks in the blockstore itself anymore (see ipfs/go-bitswap#571)

interface.go

@@ -13,9 +13,8 @@ import (
 type Interface interface { // type Exchanger interface
 	Fetcher
 
-	// TODO Should callers be concerned with whether the block was made
-	// available on the network?
-	HasBlock(context.Context, blocks.Block) error
+	// NotifyNewBlocks tells the exchange that new blocks are available and can be served.
+	NotifyNewBlocks(ctx context.Context, blocks ...blocks.Block) error
 
 	IsOnline() bool