Post and forget

[alexander-yevsyukov]

This PR adds an ability to post a command without subscribing to events.

Prior to this PR, it was possible to call CommandRequest.post() ignoring the result of the method. Now the contract is tightened:

• It is required to subscribe to at least one event before calling post(). Otherwise, IllegalStateException will be thrown suggesting to call observe() or postAndForget() instead.
• If no events are of interest for the client-side code, postAndForget() should be called. If this method is called and at least one event subscription were requested, the method throws IllegalStateException suggesting to call post() instead.
• The value returned by post() cannot be ignored.

[codecov]

Codecov Report

Merging #1292 into master will decrease coverage by 0.01%.
The diff coverage is 85.00%.

@@             Coverage Diff              @@
##             master    #1292      +/-   ##
============================================
- Coverage     91.06%   91.04%   -0.02%     
- Complexity     4708     4712       +4     
============================================
  Files           606      606              
  Lines         14992    15005      +13     
  Branches        851      853       +2     
============================================
+ Hits          13652    13662      +10     
- Misses         1072     1077       +5     
+ Partials        268      266       -2     

[yuri-sergiichuk]

@alexander-yevsyukov LGTM.

Please consider the minor suggestions below.