More unit tests for NUMEQUAL
Make RPC password resistant to timing attacks

Fixes issue#2838; this is a tweaked version of pull#2845 that
should not leak the length of the password and is more generic,
in case we run into other situations where we need
timing-attack-resistant comparisons.
Use HandleError() consistently to handle leveldb errors
Clarified the listsinceblock help test
Specifically, the fact that the command
relates to wallet transactions.
Fix non-standard disconnected transactions causing mempool orphans
Mempool consistency check
Only create signatures with even S, and verification mode to check.

To fix a minor malleability found by Sergio Lerner (reported here:
https://bitcointalk.org/index.php?topic=8392.msg1245898#msg1245898)

The problem is that if (R,S) is a valid ECDSA signature for a given
message and public key, (R,-S) is also valid. Modulo N (the order
of the secp256k1 curve), this means that both (R,S) and (R,N-S) are
valid. Given that N is odd, S and N-S have a different lowest bit.
We solve the problem by forcing signatures to have an even S value,
excluding one of the alternatives.

This commit just changes the signing code to always produce even S
values, and adds a verification mode to check it. This code is not
enabled anywhere yet. Existing tests in key_tests.cpp verify that
the produced signatures are still valid.
Merge pull request #2131 from sipa/evens

Only create signatures with even S, and verification mode to check.
Merge pull request #2876 from sipa/fixreorgcrash

Fix reorganization crash
Merge pull request #2886 from gavinandresen/rpctiming

Make RPC password resistant to timing attacks
Merge pull request #2880 from gavinandresen/test_numequal

More unit tests for OP_NUMEQUAL
Merge pull request #2891 from gavinandresen/leveldb_printerror

Use HandleError() consistently to handle leveldb errors
Merge pull request #2903 from Michagogo/listsinceblock-help

Clarified the listsinceblock help test