- changed the logging level of some messages that should not appear by
default in a debug build
- most errors in persistWorkItem() are gracefully recovered from
- fixed the handling of out of order blocks
- dropped work items with fewer blocks than what we requested
- getBestBlockNumber(): log and allow exceptions from getBlockHeaders()
- obtainBlocksFromPeer(): moved here the check for block numbers being
in sequence (from nimbus/p2p/chain.nim)
- sendMsg(): log and catch exceptions in callers