add timestamp field to block struct

[frrist]

What

This PR adds a timestamp to block, currently these timestamps are set to 0. This will be updated to use the BlockClock interface defined in #2894 once it merges. These updates will be made while completing #2886.

[codecov-io]

Codecov Report

Merging #2895 into feat/miner-worker-clock will increase coverage by <1%.
The diff coverage is 40%.

@@                   Coverage Diff                    @@
##           feat/miner-worker-clock   #2895    +/-   ##
========================================================
+ Coverage                       43%     43%   +<1%     
========================================================
  Files                          208     208            
  Lines                        12399   12411    +12     
========================================================
+ Hits                          5371    5389    +18     
+ Misses                        6217    6212     -5     
+ Partials                       811     810     -1

Impacted Files	Coverage Δ
types/block.go	82% <ø> (ø)	⬆️
commands/message.go	0% <0%> (ø)	⬆️
mining/block_generate.go	71% <100%> (ø)	⬆️
gengen/util/gengen.go	49% <100%> (ø)	⬆️
protocol/storage/miner.go	23% <0%> (ø)	⬆️
mining/worker.go	54% <0%> (ø)	⬆️
consensus/block_validation.go	0% <0%> (ø)	⬆️
protocol/storage/client.go	49% <0%> (ø)	⬆️
node/node.go	59% <0%> (ø)	⬆️
protocol/hello/hello.go	86% <0%> (+9%)	⬆️