Handshake early to detect plugin version mismatch

[chungers]

This PR is mostly a code refactor to let the RPC client error out early at construction time when there's a version mismatch.

• This allows error to happen early before actual spi RPC calls are made.
• A plugin implementation is still returned, even though future calls will fail unless the the version mismatch is addressed. We will consider adding mechanism for re-doing handshake thus allowing components to be swapped out on the fly without having to take everything down.

Signed-off-by: David Chung [email protected]

[codecov-io]

Current coverage is 65.71% (diff: 76.66%)

Merging #369 into master will increase coverage by 0.29%

@@             master       #369   diff @@
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  Files            47         47          
  Lines          2284       2301    +17   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits           1494       1512    +18   
+ Misses          624        623     -1   
  Partials        166        166          

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[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "plugin-handshake" [email protected]:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353454672
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.