ui: Clarify help messages for 'message' fields

Add or amend tooltips to clarify what the message is for and that it
doesn't go over the network.
Unit test fixes for AssertLockHeld / -DDEBUG_LOCKORDER

Unit tests would fail if compiled with -DDEBUG_LOCKORDER (AssertLockHeld()
would fail; AssertLockHeld() relies on the DEBUG_LOCKORDER code to keep
track of locks held).

Fixed by LOCK'ing the wallet mutex in the unit tests that manipulate the
wallet.
Fix off-by-one errors in use of IsFinalTx()

Previously CreateNewBlock() didn't take into account the fact that
IsFinalTx() without any arguments tests if the transaction is considered
final in the *current* block, when both those functions really needed to
know if the transaction would be final in the *next* block.

Additionally the UI had a similar misunderstanding.

Also adds some basic tests to check that CreateNewBlock() is in fact
mining nLockTime-using transactions correctly.

Thanks to Wladimir J. van der Laan for rebase.

Conflicts:
	src/miner.cpp
[Qt] harmonize strings used when exporting in addressbookpage
Progress