Review Request 57104: Updated a few functions in the agent to be declared `const`.

[Michael Park]

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Review request for mesos and Benjamin Mahler.


Bugs: MESOS-6627
    https://issues.apache.org/jira/browse/MESOS-6627


Repository: mesos


Description
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See summary.


Diffs
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  src/slave/slave.hpp 3b0aea4e3e9a17501077beccbccaab4abbe11af2 
  src/slave/slave.cpp fc480ae23ffa5cdeeb79b3621a08e1f8703bc01a 

Diff: https://reviews.apache.org/r/57104/diff/


Testing
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`make check`


Thanks,

Michael Park