overhaul serialization code

The implementation of each class' serialization/deserialization is no longer
passed within a macro. The implementation now lies within a template of form:

template <typename T, typename Stream, typename Operation>
inline static size_t SerializationOp(T thisPtr, Stream& s, Operation ser_action, int nType, int nVersion) {
    size_t nSerSize = 0;
    /* CODE */
    return nSerSize;
}

In cases when codepath should depend on whether or not we are just deserializing
(old fGetSize, fWrite, fRead flags) an additional clause can be used:
bool fRead = boost::is_same<Operation, CSerActionUnserialize>();

The IMPLEMENT_SERIALIZE macro will now be a freestanding clause added within
class' body (similiar to Qt's Q_OBJECT) to implement GetSerializeSize,
Serialize and Unserialize. These are now wrappers around
the "SerializationOp" template.
remove fields of ser_streamplaceholder

The nType and nVersion fields of stream objects are never accessed
from outside the class (or perhaps from the inside too, I haven't checked).
Thus no need to have them in a placeholder, whose only purpose is to
fill the "Stream" template parameter in serialization implementation.
rework overhauled serialization methods to non-static

Thanks to Pieter Wuille for most of the work on this commit.
I did not fixup the overhaul commit, because a rebase conflicted
with "remove fields of ser_streamplaceholder".
I prefer not to risk making a mistake while resolving it.
Use CSizeComputer to avoid counting sizes in SerializationOp
Merge pull request #4737

31e9a83 Use CSizeComputer to avoid counting sizes in SerializationOp (Pieter Wuille)
84881f8 rework overhauled serialization methods to non-static (Kamil Domanski)
5d96b4a remove fields of ser_streamplaceholder (Kamil Domanski)
3d796f8 overhaul serialization code (Kamil Domanski)