Use mutex pointer instead of name for AssertLockHeld

This makes it useable for non-global locks such as the wallet and
keystore locks.
Document cs_wallet lock and add AssertLockHeld

Add locking assertions to wallet to all methods that
access internal fields and do not aquire the cs_wallet mutex.
LoadWallet: acquire cs_wallet mutex before clearing setKeyPool

Make the function mutex-aware, to prevent having to lock cs_wallet
at the call site in Init.
Changed Get64(.) to GetLow64()

The function Get64(.) has a bug in case the width is not divisible by 64.
Since it is only ever used as Get64(0) this simply changes it to this
special case. Additionally, an assert is added, and a cast to prevent
a compiler error.
Unittests for uint256.h

Unit tests for uint256.h. The file uint160_tests.cpp is no longer
needed. The ad-hoc tests which were in uint256.h are also no longer
needed. The new tests achieve 100% coverage.
Update debian/
Merge pull request #3468

4d46da2 Update debian/ (Matt Corallo)
Merge pull request #3422

daa6b20 Unittests for uint256.h (Thomas Holenstein)
e85e19b Changed Get64(.) to GetLow64() (Thomas Holenstein)
Merge pull request #3401

012ca1c LoadWallet: acquire cs_wallet mutex before clearing setKeyPool (Wladimir J. van der Laan)
9569168 Document cs_wallet lock and add AssertLockHeld (Wladimir J. van der Laan)
19a5676 Use mutex pointer instead of name for AssertLockHeld (Wladimir J. van der Laan)
qt: add missing cs_wallet lock in AddressTableModel::setData

duplicate check in AddressTableModel::setData accesses
wallet data structure as well as SetAddressBook without proper LOCK, fix this.
qt: Add missing LOCKs for locked coin functions

These don't aquire the wallet lock internally, so the caller has to do
it.
qt: protect SetAddressBook with cs_wallet lock everywhere
qt: Add missing lock in WalletModel::listCoins

Another problem detected by cs_wallet lock detection (#3401).
Merge pull request #3413

d31ad26 qt: Add missing lock in WalletModel::listCoins (Wladimir J. van der Laan)
28352af qt: protect SetAddressBook with cs_wallet lock everywhere (Wladimir J. van der Laan)
aaf8d15 qt: Add missing LOCKs for locked coin functions (Wladimir J. van der Laan)
4757e92 qt: add missing cs_wallet lock in AddressTableModel::setData (Wladimir J. van der Laan)