Add variable length integers for pings

[lenrik1589]

This PR implements new data type for pings: variable length unsigned integers which allow sending integers in ranges:

• -64 to 63 using 1 byte
• -8192 to 8191 using 2 bytes
• -1048576 to 1048577 using 3 bytes
• -2147483647 to 2147483648 using 4 bytes

(all ranges inclusive, byte representing type not included)