Checkpoint message binary format

#	Field name	Type	Length in Bytes
1	Packet length (BigEndian)	Int	4
2	Magic Bytes	Bytes	4
3	Content ID (0x64)	Byte	1
4	Payload length	Int	4
5	Payload checksum	Bytes	4
6	Checkpoint items count (N)	Int	4
7	Checkpoint #1 height	Long	8
8	Checkpoint #1 signature	Bytes	64
...	...	...	...
6 + 2 * N - 1	Checkpoint #N height	Long	8
6 + 2 * N	Checkpoint #N signature	Bytes	64