[#19] support for checkpoint files
[#19] make checkpoint file path configurable

the checkpoint file will be something bundled with the application, so
it will normally be located in a completely different place than the
wallet and spv files, which are generated in user's data folder by the
application
[#19] ask wallet about the earliest key creation time
[#19] use time of first Hive commit as fallback
[#19] manually preserve creation time in ECKeys