Events export and import

[uprendis]

• Add export command
• • Dumps current events into a file, parents are always ordered before children
• • Creates an events file in specified location
• • Events file has a header and version
• • Optionally, only a specified range of epochs may be dumped
• Add import command
• • Imports events from specified events file(s)
• • Events are fully verified by default, unless overridden by check=false flag. If events are fully verified, then the process is trustless.
• • Import of events with full verification takes ~2 times longer than without full verification.

Re-work of #481 for develop2 branch