Add support for passing CTransaction and CTransactionRef types to IPC
functions.
These types can't be passed currently because IPC serialization code currently
only supports deserializing types that have an Unserialize() method, which
CTransaction does not, because it is supposed to represent immutable
transactions. Work around this by adding a CustomReadField overload that will
call CTransaction's deserialize_type constructor.
These types also can't be passed currently because serializing transactions
requires TransactionSerParams to be set. Fix this by setting TX_WITH_WITNESS as
default serialization parameters for IPC code.
Allow any C++ object that has Serialize and Unserialize methods and can be
serialized to a bitcoin CDataStream to be converted to a capnproto Data field
and passed as arguments or return values to capnproto methods using the Data
type.
Extend IPC unit test to cover this and verify the serialization happens
correctly.
Add unit test to test IPC method calls and type conversion between bitcoin c++
types and capnproto messages.
Right now there are custom type hooks in bitcoin IPC code, so the test is
simple, but in upcoming commits, code will be added to convert bitcoin types to
capnproto messages, and the test will be expanded.
