Add multinode test for two nodes rotating at 1 tick per slot

[mvines]

Shows that two nodes can rotate at 2 ticks per slot fine, but at 1 ticks per slot two nodes fall down -- need debug still

[codecov]

Codecov Report

Merging #2735 into master will decrease coverage by <.1%.
The diff coverage is 83.3%.

@@           Coverage Diff            @@
##           master   #2735     +/-   ##
========================================
- Coverage    79.1%   79.1%   -0.1%     
========================================
  Files         115     115             
  Lines       19185   19183      -2     
========================================
- Hits        15177   15174      -3     
- Misses       4008    4009      +1

[mvines]

I think this works now! Let's see if CI agrees. The problems were:

1. Off-by-one in fullnode.rs
2. It's too hard to create a valid ledger with blocktree::write_entries(). Some minor tweaks were needed to the test that built on some of other earlier work today in blocktree. But overall the problem still remains, blocktree::write_entries() needs more TLC