Update key fields encode/decode into static func

Upstream commit ID : fb-mysql-5.6.35/c470e31144ffec8ba228b4272ce378e2ed4192d1
PS-6865 : Merge fb-prod201903

Summary:
Currently, all index(key) fields' pack(encode)/unpack(decode)/skip functions are member functions of type Rdb_key_def, due to varchar field pack/unpack need to know whether to use legacy varchar encoder/decoder or not from Rdb_key_def instance.

The change is to update all index(key) encode/decode function from member function to static function by storing whether index(key) field use legacy varchar encoder/decoder or not into Rdb_field_packing field.

Reviewed By: yizhang82

Differential Revision: D14594041

fbshipit-source-id: c3ec20b25eb