Review Request 42182: Removed the erroneous check that did not allow HTTP based executors to subscribe upon agent process restart.

[Anand Mazumdar]

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Review request for mesos and Vinod Kone.


Bugs: MESOS-4255
    https://issues.apache.org/jira/browse/MESOS-4255


Repository: mesos


Description
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Previously, we used to return a `ServiceUnavailable` when slave was recovering. 
However, we only need to do so, for all other calls and let `Subscribe` pass 
through.


Diffs
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  src/slave/http.cpp 66abdcb12d670bc4b77fa5b5f8ce35220f9aa698 
  src/slave/slave.cpp 90d0fecd2d83fd174134870a577ac59d79c0006f 

Diff: https://reviews.apache.org/r/42182/diff/


Testing
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make check


Thanks,

Anand Mazumdar