Use ModifyCoins instead of mutable GetCoins.

Replace the mutable non-copying GetCoins method with a ModifyCoins, which
returns an encapsulated iterator, so we can keep track of concurrent
modifications (as iterators can be invalidated by those) and run cleanup
code after a modification is finished.

This also removes the overloading of the 'GetCoins' name.
Get rid of CCoinsView's SetCoins and SetBestBlock.

All direct modifications are now done through ModifyCoins, and BatchWrite is
used for pushing batches of queued modifications up, so we don't need the
low-level SetCoins and SetBestBlock anymore in the top-level CCoinsView class.
Do not keep fully spent but unwritten CCoins entries cached.

Instead of storing CCoins entries directly in CCoinsMap, store a CCoinsCacheEntry
which additionally keeps track of whether a particular entry is:
* dirty: potentially different from its parent view.
* fresh: the parent view is known to not have a non-pruned version.

This allows us to skip non-dirty cache entries when pushing batches of changes up,
and to remove CCoins entries about transactions that are fully spent before the
parent cache learns about them.
Add coins_tests with a large randomized CCoinViewCache test.
Get rid of the dummy CCoinsViewCache constructor arg
fix a possible memory leak in CWalletDB::Recover

- convert pdbCopy into a boost::scoped_ptr to ensure memory gets freed
  in all cases (e.g. after "ret > 0")
fix possible memory leaks in CWallet::EncryptWallet

- add missing deletes for pwalletdbEncryption
- add an assert before trying to reserve memory for pwalletdbEncryption
- add a destructor to CWallet, which ensures deletion of
  pwalletdbEncryption on object destruction
change exit(1) to an assert in CWallet::EncryptWallet
Merge pull request #4834

7c70438 Get rid of the dummy CCoinsViewCache constructor arg (Pieter Wuille)
ed27e53 Add coins_tests with a large randomized CCoinViewCache test. (Pieter Wuille)
058b08c Do not keep fully spent but unwritten CCoins entries cached. (Pieter Wuille)
c9d1a81 Get rid of CCoinsView's SetCoins and SetBestBlock. (Pieter Wuille)
f28aec0 Use ModifyCoins instead of mutable GetCoins. (Pieter Wuille)
Make SCRIPT_VERIFY_STRICTENC compatible with BIP62

* Delete canonical_tests.cpp, and move the tests to script_tests.cpp.
* Split off SCRIPT_VERIFY_DERSIG from SCRIPT_VERIFY_STRICTENC (the BIP62 part of it).
* Change signature STRICTENC/DERSIG semantics to fail the script entirely rather than the CHECKSIG result (softfork safety, and BIP62 requirement).
* Add many autogenerated tests for several odd cases.
* Mention specific BIP62 rules in the script verification flags.
Merge pull request #5004

9df9cf5 Make SCRIPT_VERIFY_STRICTENC compatible with BIP62 (Pieter Wuille)
Merge pull request #4996

d0c4197 change exit(1) to an assert in CWallet::EncryptWallet (Philip Kaufmann)
870da77 fix possible memory leaks in CWallet::EncryptWallet (Philip Kaufmann)
f606bb9 fix a possible memory leak in CWalletDB::Recover (Philip Kaufmann)