[Qt] add BIP70/BIP71 constants for all messages and mime types

- also rename current ones to match the new ones
- remove constant from guiconstant.h and add it to paymentserver.cpp
[Qt] ensure socket is set to NULL in PaymentServer::ipcSendCommandLine
[Qt] remove dup lock that is done in SetAddressBook()
[Qt] add BIP70 payment request size DoS protection for URIs

- current code only does this for payment request files, which are
  used on Mac
- also rename readPaymentRequest to readPaymentRequestFromFile, so it's
  obvious that function only handles payment request files and not URIs
- small logging changes in readPaymentRequestFromFile