Don't assign a transaction type as a private variable

[MaciejBaj]

Expected behavior

Transaction type to compare the type against, for example const TRANSACTION_TRANSFER_TYPE = 0; for type 0 transaction is assigned in a way that can be overridden by the transactions that inherit after BaseTransction.

An example would be implementing the valid type as a static field:

class TransferTransaction {
   protected static TYPE = 10;
 ...

Actual behavior

The valid type is assigned as a private variable. Each function comparing a type (validate) needs to be re-implemented and cannot be overridden if a BaseTransction cannot access a valid transaction type.

Which version(s) does this affect? (Environment, OS, etc...)

2.0.0