[Java] Fast string serialization support

[chaokunyang]

Is your feature request related to a problem? Please describe.
String is very common in serialization, but due to its variable length and mutiple encoding, string serialization is pretty slow, sometimes is becomes the bottle of whole serialization. We need a way for fast string serialization.

The bottle mainly consists of:

• String data serialization copy: copy inner char[] / byte[] outside for serialization.
• String encoding: encoding char[] / byte[] into ascii/unicode16/utf8
• String decoding: decoding binary into ascii/unicode16 char[]/byte[]
• String creation copy cost: java.lang.String will copy provided char[]/byte[] for immutability.

Describe the solution you'd like

• Use sun.misc.Unsafe for extract inner char[] / byte[]
• Support ascii/unicode16/utf8 to minimize encoding cost
• Add encoding flag in data to support multiple encoding
• Use java.lang.invoke.MethodHandle to avoid invoke package-level zero0-copy constructor with minimal cost