MXRestClient: Factorise processing and completion blocks handling
MXRestClient: Factorise processing and completion blocks handling

and kill all "Block implicitly retains self" warnings (https://github.com/vector-im/riot-ios/issues/1898)

matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk