Speedup np-import when appending blocks

[ixje]

What current issue(s) does this address, or what feature is it adding?
when we supplied the -a flag for appending blocks, the code would still deserialise all data

How did you solve this problem?
skip block deserialization and move the file pointer directly to the next block until we've reached the current height

How did you make sure your solution works?
manual testing

Are there any special changes in the code that we should be aware of?

Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[coveralls]

Coverage remained the same at 85.113% when pulling ab72804 on ixje:speedup-import into 82594df on CityOfZion:development.