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)