c++11 (libc++'s stdlib implementation anyway) doesn't allow for map types to be
forward-declared. for example:

class foo;
std::map<int, foo> bar; // error, foo has not been defined.
class foo{};

Since CWallet and CWalletTx are inter-dependent, but only std::map<*,CWalletTx>
is used, forward-declare CWallet instead and define CWalletTx first.

Despite the mangled git diff, this change only amounts to moving ~320 lines in
a single chunk.