Fix block producers forking on startup

[leventeliu]

Fixes

1. Move limits package to conf.
2. Fix a height overflow issue while calculating with time before genesis time.
3. Add a parameter to configure required reachable BPs for genesis startup.

Notes

Configuring a conf.BPStartupRequiredReachableCount which is greater than 1 will require the genesis time to be in the future for a genesis startup, otherwise, no BP will be able to move forward.
And it also requires at least conf.BPStartupRequiredReachableCount-1 BP(s) to be alive at any time, or the BP network will stop working forever.

[codecov]

Codecov Report

Merging #211 into develop will decrease coverage by 0.12%.
The diff coverage is 80.55%.

@@             Coverage Diff             @@
##           develop     #211      +/-   ##
===========================================
- Coverage    76.27%   76.15%   -0.13%     
===========================================
  Files          136      136              
  Lines        11179    11188       +9     
===========================================
- Hits          8527     8520       -7     
- Misses        1991     2005      +14     
- Partials       661      663       +2

[codecov]

Codecov Report

Merging #211 into develop will decrease coverage by 0.05%.
The diff coverage is 81.08%.

@@             Coverage Diff             @@
##           develop     #211      +/-   ##
===========================================
- Coverage    76.27%   76.22%   -0.06%     
===========================================
  Files          136      136              
  Lines        11179    11188       +9     
===========================================
+ Hits          8527     8528       +1     
- Misses        1991     2000       +9     
+ Partials       661      660       -1

[xq262144]

Approve

[auxten]

LGTM