server: add -r option on startup to specify the initial role

[dbkernel]

We don't want to affect MySQL services when we upgrade xenon, so after the upgrade need to let the old LEADER continue to run as the LEADER and skip leaderStartShellCommand.

[codecov[bot]]

Codecov Report

Merging #126 (44725d4) into master (6649cea) will decrease coverage by 0%. The diff coverage is 95%.

:exclamation: Current head 44725d4 differs from pull request most recent head 809a19a. Consider uploading reports for the commit 809a19a to get more accurate results

@@          Coverage Diff          @@
##           master   #126   +/-   ##
=====================================
- Coverage      68%    68%   -1%     
=====================================
  Files          48     48           
  Lines        3721   3742   +21     
=====================================
+ Hits         2533   2547   +14     
- Misses        954    960    +6     
- Partials      234    235    +1     

Impacted Files	Coverage Δ
src/mysql/api.go	61% <0%> (-1%)	:arrow_down:
src/raft/leader.go	84% <80%> (-2%)	:arrow_down:
src/config/config.go	80% <100%> (+<1%)	:arrow_up:
src/raft/mock.go	83% <100%> (+<1%)	:arrow_up:
src/raft/raft.go	84% <100%> (+1%)	:arrow_up:
src/server/mock.go	100% <100%> (ø)
src/server/server.go	51% <100%> (ø)
src/raft/candidate.go	83% <0%> (-1%)	:arrow_down:

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[andyli029]

Xenon Coverage / Coverage Failed. @drdstech

[dbkernel]

push update:
fix TestRaftLeaderChangeToMasterError failed

[andyli029]

Split the pr into several prs.

[dbkernel]

push update:

1. split pr
2. add doc and comments

[andyli029]

Ping @hustjieke