Fix deadlocks on failures with parTraverse and parSequence

[ngbinh]

Both of these tests fail by timing out on my local machine:

[info] BlazeClientSpec                                                                                     
[info] Blaze Http1Client should                                     
[info]   + raise error NoConnectionAllowedException if no connections are permitted for key (789 ms)
[info]   + make simple https requests (966 ms)                                                                                                      
[info]   + behave and not deadlock (1 second, 34 ms)                                         
[error]   x behave and not deadlock on failures with parTraverse (30 seconds, 245 ms)                      
[error]    None is not Some (BlazeClientSpec.scala:151)                               
[error] org.http4s.client.blaze.BlazeClientSpec.$anonfun$new$18(BlazeClientSpec.scala:151)
[error]   x behave and not deadlock on failures with parSequence (30 seconds, 308 ms)                              
[error]    None is not Some (BlazeClientSpec.scala:184)                                                                                  
[error] org.http4s.client.blaze.BlazeClientSpec.$anonfun$new$31(BlazeClientSpec.scala:184)                                                                                                                  
[info]   + obey response header timeout (30 seconds, 374 ms)                                                                                                                                                
[info]   + unblock waiting connections (30 seconds, 417 ms)                                                                                                       
[info]   + reset request timeout (30 seconds, 462 ms)                  
[info]   + drain waiting connections after shutdown (1 second, 140 ms)                                                                   
[info]   + cancel infinite request on completion (1 second, 193 ms)                                                                                                                                         
[info] Total for specification BlazeClientSpec                  
[info] Finished in 31 seconds, 904 ms                    
[info] 10 examples, 38 expectations, 2 failures, 0 error  

[ngbinh]

@rossabaker should this be reopened?

[rossabaker]

Yes, I mentioned it in the other ticket and it autoclosed.

[rossabaker]

After 212d7fa, the parTraverse test still fails for me due to #2265. This fixes the deadlocks.

[ngbinh]

I remember there are cases where we need to flatmap two failures to reproduce the error.

[rossabaker]

Travis is a bit underpowered, so hitting it too hard can lead to spurious failures, so we'll have to find the right balance here. I'll bump it back up, and we'll see if we can turn it back down.

It passed, when it fails most of the time for me locally on #2265. Some bugs are exposed only by slow Travis and some bugs are hidden by slow Travis. 😩

[rossabaker]

The last commit should also fix #2265.

[ngbinh]

looks great to me. Thanks!