[backport: 17.06] [manager/dispatcher] Replace call to isRunning() to isRunningLocked() in dispatcher Heartbeat()

[anshulpundir]

backport of #2664 for 17.06

git cherry-pick -s -S -x caee4da
cherry-pick was not clean.

We noticed repeated CI failures pointing to data races because of unlocked access to the dispatcher context in Heartbeat(). Golang also does not provide any guarantees around read-only operations on objects which are otherwise locked.

This will likely have a performance impact, which we will evaluate and some of the dispatcher code might need to be re-written accordingly. But correctness is first.

[codecov]

Codecov Report

Merging #2715 into bump_v17.06 will increase coverage by 0.41%.
The diff coverage is 0%.

@@               Coverage Diff               @@
##           bump_v17.06    #2715      +/-   ##
===============================================
+ Coverage        60.87%   61.28%   +0.41%     
===============================================
  Files               30      121      +91     
  Lines             4322    20203   +15881     
===============================================
+ Hits              2631    12382    +9751     
- Misses            1435     6462    +5027     
- Partials           256     1359    +1103

[cyli]

LGTM thank you!