Add HaveInventory function.

This commit provides a new exported function, HaveInventory, which can
be used to determine if the already has the item referenced by passed
inventory vector.  Currently it only provides support for blocks and
cursory support for transactions in the main chain.  Types that are
unrecognized are returned as if they unknown as expected.

chain.go

@@ -168,6 +168,40 @@ func (b *BlockChain) DisableVerify(disable bool) {
 	b.noVerify = disable
 }
 
+// HaveInventory returns whether or not the chain instance has the inventory
+// represented by the passed inventory vector.  This includes checking all of
+// the various places inventory can be when it is in different states such as
+// part of the main chain, on a side chain, and orphans.
+//
+// This function is safe for concurrent access.
+func (b *BlockChain) HaveInventory(inventoryVector *btcwire.InvVect) bool {
+	switch inventoryVector.Type {
+	case btcwire.InvVect_Block:
+		// Check the main chain and side chains.
+		if b.blockExists(&inventoryVector.Hash) {
+			return true
+		}
+
+		// Check orphan blocks.
+		if b.IsKnownOrphan(&inventoryVector.Hash) {
+			return true
+		}
+
+	case btcwire.InvVect_Tx:
+		// TODO(davec): Need to ultimately maintain a transaction pool
+		// of transactions that are not already in a block and check
+		// for the existing transaction there too.
+
+		// Check if the transaction exists from the point of view of the
+		// end of the main chain.
+		return b.db.ExistsTxSha(&inventoryVector.Hash)
+	}
+
+	// The requested inventory is either not known or is an unsupported
+	// type (which also implies it is not known).
+	return false
+}
+
 // IsKnownOrphan returns whether the passed hash is currently a known orphan.
 // Keep in mind that only a limited number of orphans are held onto for a
 // limited amount of time, so this function must not be used as an absolute