Allow import or export blocks

[erikzhang]

No description provided.

[vncoelho]

Looks good, @erikzhang, I do know yet if this will be as flexible as the plugin. however, it makes sense to be merged here.

[cloud8little]

@vncoelho @erikzhang I will have a test first and please hold on the merging.

[cloud8little]

Test scenarios all passed:
neo-cli: master #501 + pr 502
neo: 3.0.0-CI00817

os: ubuntu 18.04
current best height: 533

1. export block Pass

neo> export block
[533/533]

2. export block 0 30 Pass
3. export block 30 Pass
4. export block 0 30 -> export block 30 100, delete seed node in protocol.json, the highest block reach to 129

neo> export block 0 30
[29/29]
neo> export block 30 100
[129/129]

5. current best height: 414 Pass

neo> export block 450
neo> export block 0 -1
error: Value was either too large or too small for a UInt32.
neo> export block 410 -1
error: Value was either too large or too small for a UInt32. 

[erikzhang]

@cloud8little Merge?

[cloud8little]

Good to merge from QA's perspective.