significant optimizations to Solver and CWalletTx::GetAmounts
a few more tweaks
alternate listaccounts / GetCredits functions
add caching to CWalletTx::GetAmounts and undo some other changes
Merge branch 'master-1.5' into optim2-1.5
Merge pull request #274 from somegeekintn/optim2-1.5

Wallet/RPC Optimizations