Maintenance update

[OlegDokuka]

• Clean-up immutability for Sender and Receiver options

• migrate to kafka 2.0.0

• update gradle to 4.10

• removed redundant KafkaScheduler#fromWorker (replaced to Schedulers.single())

• update to Reactor 3.2.0.RELEASE

• update docs to upcoming release

[codecov-io]

Codecov Report

Merging #65 into master will decrease coverage by 3.64%.
The diff coverage is 62.5%.

@@             Coverage Diff              @@
##             master      #65      +/-   ##
============================================
- Coverage     84.73%   81.08%   -3.65%     
- Complexity      192      228      +36     
============================================
  Files            14       18       +4     
  Lines          1022     1121      +99     
  Branches        111      113       +2     
============================================
+ Hits            866      909      +43     
- Misses          120      174      +54     
- Partials         36       38       +2

Impacted Files	Coverage Δ	Complexity Δ
...ctor/kafka/receiver/internals/KafkaSchedulers.java	81.39% <ø> (+1.68%)	3 <0> (ø)	⬇️
...eactor/kafka/sender/internals/ProducerFactory.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...tor/kafka/sender/internals/DefaultKafkaSender.java	89.38% <100%> (+0.14%)	32 <0> (+1)	⬆️
...ctor/kafka/receiver/internals/ConsumerFactory.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...kafka/receiver/internals/DefaultKafkaReceiver.java	88.96% <100%> (+1.14%)	59 <0> (ø)	⬇️
...actor/kafka/receiver/ImmutableReceiverOptions.java	45.53% <45.53%> (ø)	23 <23> (?)
.../main/java/reactor/kafka/sender/SenderOptions.java	54.54% <55.55%> (-11.61%)	8 <6> (-14)
...reactor/kafka/receiver/MutableReceiverOptions.java	67.92% <67.92%> (ø)	36 <36> (?)
...ava/reactor/kafka/sender/MutableSenderOptions.java	70% <70%> (ø)	14 <14> (?)
...a/reactor/kafka/sender/ImmutableSenderOptions.java	70.73% <70.73%> (ø)	11 <11> (?)
... and 7 more

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[smaldini]

With package private changes

[simonbasle]

The commit corresponding to this PR seems to have been tagged with 1.1.0.RELEASE manually by @OlegDokuka.

Yet the 1.1.0 milestone still has 6 issues open, and this tag doesn't match the usual format (v prefix) nor process (spring-buildmaster), plus I can't find any trace of a proper release anywhere.

So it begs the question: what is the status of 1.1.0? Am I expected to properly release the master branch and move all open issues to a further milestone? Or should Californium-SR1 stay on 1.0.1.RELEASE?

⚠️ This is blocking the Californium-SR1 release today ⚠️