Implement --join to bring up multi-node cluster

[candlerb]

No description provided.

[codecov]

Codecov Report

Merging #136 into master will increase coverage by 28.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #136       +/-   ##
===========================================
+ Coverage    41.4%   69.44%   +28.03%     
===========================================
  Files          84        9       -75     
  Lines        5936      576     -5360     
===========================================
- Hits         2458      400     -2058     
+ Misses       3022      114     -2908     
+ Partials      456       62      -394

Impacted Files	Coverage Δ
protocol/metadata_request.go
protocol/offset_commit_request.go
protocol/size_field.go
protocol/response.go
protocol/request.go
jocko/fsm/index.go
protocol/heartbeat_response.go
protocol/sync_group_request.go
protocol/list_groups_request.go
protocol/error.go
... and 65 more

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