@russell/storage upload task snapshot

[russellwheatley]

fixes #3250

• Switched put method to return a StorageTask, not a promise. This is problem as it stops the user being able to use StorageTask functionality.

• Updates storage Task so it has a snapshot property available as per the web spec: https://firebase.google.com/docs/reference/js/firebase.storage.UploadTask#snapshot
  Value is initially set to null which is different to the web spec.

I tried using the promiseDefer method for the then getter property in StorageTask, but (as far as I I've tried), it doesn't work for our purposes.

[codecov]

Codecov Report

Merging #3330 into master will increase coverage by 3.62%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3330      +/-   ##
==========================================
+ Coverage   81.55%   85.17%   +3.62%     
==========================================
  Files         108      108              
  Lines        3409     3425      +16     
==========================================
+ Hits         2780     2917     +137     
+ Misses        629      508     -121

[Salakar]

Linting failure on CI;

[Salakar]

Also this has all the commits from next branch attached to it, you'll have to manually apply the changes to a new local branch based off of master I guess

[russellwheatley]

Yeah, you're right. I'll create another branch then