Bitcoin-Qt: never display own block count > estimated block count

- some users reported is as weird, that the estimated block count could be
  lower than our own nodes block number
- this pull adds a new default behaviour, which displays our own block
  number as estimated block number, if own >= est. block count
- the pull raises space for nodes block counts in cPeerBlockCounts to 8 to
  be more accurate
- also removes a reduntant setNumBlocks() call in RPCConsole and moves
  initialisation of numBlocksAtStartup in ClientModel, where it belongs