Add options to _linkWith(provider) for masterKey

[acinader]

1. without the master key it is not possible to link a user with an auth provider in cloud code
2. remove a recent addition to pass the sessionToken which was added for anonymous auth, but not necessary and causing problems. Also remove the related test.

[acinader]

@dplewis parse-server unit tests, my cloud code unit tests and the specific issue I was having (linking a user to a custom OAuth provider in cloud) all work with these changes, but I want to cover and provide passing tests with the master key and test that throw when there is no master key, but I am having trouble getting it to work. gonna sleep on it. If you see anything obvious or have any guidance, I'm all ears :).

[dplewis]

@acinader Thanks for getting this started. I fixed the tests.

On the client side - you can link with current user / sessionToken
On server side - since current user doesn't exist you have to use master key / sessionToken

I realized the docs needs to be updated for this. Unlinking

[codecov]

Codecov Report

Merging #767 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #767     +/-   ##
=========================================
+ Coverage   90.37%   90.57%   +0.2%     
=========================================
  Files          54       54             
  Lines        4905     4903      -2     
  Branches     1089     1088      -1     
=========================================
+ Hits         4433     4441      +8     
+ Misses        472      462     -10

Impacted Files	Coverage Δ
src/ParseObject.js	90.02% <ø> (+0.09%)	⬆️
src/ParseUser.js	82.25% <100%> (+2.15%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b5e51d3...155310d. Read the comment docs.