[Java] use varint for class id to reduce space cost

[chaokunyang]

Is your feature request related to a problem? Please describe.

Currently class id is written as 3 bytes:

| flag byte | two bytes short class id |

This is wasteful for space. For string/int/long/etc. classes, class id can be written using 1 bytes.

With this PR, class id will be

Describe the solution you'd like

New class id encoding format:

• use varint for class id encoding to reduce java class writing space cost.
• Use little endian order for class id encoding
• Use bit 0 as flag to indicate the class will be written using class id
• When reading, read one byte first, since writting use little endian, the bit 0 of read byte can be used as a flag to determine whether this class is written using class id. If so, read varint and >>1 to get real class id. Otherwise, read class by classname instead.