DEBUG_LOCKORDER : early detection of potential deadlocks

Commit 865ed8a adds a new compile-time #ifdef : DEBUG_LOCKORDER

Compile with -DDEBUG_LOCKORDER and every time a lock is acquired by a CCriticalSection a check is made to record the order of locks and complain if they are being acquired in an inconsistent order. Here's what you get in debug.log when potential deadlocks are detected:

POTENTIAL DEADLOCK DETECTED Previous lock order was: pwallet->cs_mapWallet db.cpp:686 pwallet->cs_KeyStore db.cpp:687 cs_KeyStore keystore.cpp:74 cs_vMasterKey keystore.cpp:75 cs_KeyStore keystore.cpp:31 Current lock order is: pwallet->cs_mapWallet db.cpp:686 pwallet->cs_KeyStore db.cpp:687 cs_KeyStore keystore.cpp:74 cs_vMasterKey keystore.cpp:75

POTENTIAL DEADLOCK DETECTED Previous lock order was: pnode->cs_vRecv net.cpp:1525 cs_main main.cpp:2372 cs_vSend net.h:681 Current lock order is: pnode->cs_vSend net.cpp:1531 cs_main main.cpp:2411

I'll be working on figuring out how to eliminate these, and will be working on some RPC stress-tests that try to exercise all the code paths to early-detect other potential problems.