Speed up RPC authentication (reworked pull from Joel Katz)
Faster Base64 decoder.
Use transactions-updated as flush condition

The normal checkpointing during the block chain download is reduced
to every five minutes only, but forced every 200000 updated transactions.

Backported from bitcoin commit 25c5eca8939832e94556810ae30f1f7830e1c29c
Flush on log size instead of transaction count

Backported from bitcoin commit e88ed65a39eb86ffd4d0720852d7e5f3d021f792
Fix compilation warning
Merge pull request #25 from khalahan/speed-improvements

Speed improvements