Cleanup code using forward declarations.

Use misc methods of avoiding unnecesary header includes.
Replace int typedefs with int##_t from stdint.h.
Replace PRI64[xdu] with PRI[xdu]64 from inttypes.h.
Normalize QT_VERSION ifs where possible.
Resolve some indirect dependencies as direct ones.
Remove extern declarations from .cpp files.

src/txmempool.h

@@ -6,6 +6,7 @@
 #define BITCOIN_TXMEMPOOL_H
 
 #include "core.h"
+#include "sync.h"
 
 /*
  * CTxMemPool stores valid-according-to-the-current-best-chain