Add new funcs to allow size hints on getdata/inv.

This commit adds two new functions named NewMsgGetDataSizeHint and
NewMsgInvSizeHint.  These are intended to allow callers which know in
advance how large the inventory lists will grow the ability to provides
that information when creating the message.  This in turn provides a
mechanism to avoid the need to perform several grow operations of the
backing array when adding large number of inventory vectors.

msggetdata.go

@@ -107,3 +107,24 @@ func NewMsgGetData() *MsgGetData {
 		InvList: make([]*InvVect, 0, defaultInvListAlloc),
 	}
 }
+
+// NewMsgGetDataSizeHint returns a new bitcoin getdata message that conforms to
+// the Message interface.  See MsgGetData for details.  This function differs
+// from NewMsgGetData in that it allows a default allocation size for the
+// backing array which houses the inventory vector list.  This allows callers
+// who know in advance how large the inventory list will grow to avoid the
+// overhead of growing the internal backing array several times when appending
+// large amounts of inventory vectors with AddInvVect.  Note that the specified
+// hint is just that - a hint that is used for the default allocation size.
+// Adding more (or less) inventory vectors will still work properly.  The size
+// hint is limited to MaxInvPerMsg.
+func NewMsgGetDataSizeHint(sizeHint uint) *MsgGetData {
+	// Limit the specified hint to the maximum allow per message.
+	if sizeHint > MaxInvPerMsg {
+		sizeHint = MaxInvPerMsg
+	}
+
+	return &MsgGetData{
+		InvList: make([]*InvVect, 0, sizeHint),
+	}
+}

msginv.go

@@ -115,3 +115,24 @@ func NewMsgInv() *MsgInv {
 		InvList: make([]*InvVect, 0, defaultInvListAlloc),
 	}
 }
+
+// NewMsgInvSizeHint returns a new bitcoin inv message that conforms to the
+// Message interface.  See MsgInv for details.  This function differs from
+// NewMsgInv in that it allows a default allocation size for the backing array
+// which houses the inventory vector list.  This allows callers who know in
+// advance how large the inventory list will grow to avoid the overhead of
+// growing the internal backing array several times when appending large amounts
+// of inventory vectors with AddInvVect.  Note that the specified hint is just
+// that - a hint that is used for the default allocation size.  Adding more
+// (or less) inventory vectors will still work properly.  The size hint is
+// limited to MaxInvPerMsg.
+func NewMsgInvSizeHint(sizeHint uint) *MsgInv {
+	// Limit the specified hint to the maximum allow per message.
+	if sizeHint > MaxInvPerMsg {
+		sizeHint = MaxInvPerMsg
+	}
+
+	return &MsgInv{
+		InvList: make([]*InvVect, 0, sizeHint),
+	}
+}