Enable errorprone compiler plugin in build

[lhotari]

• enable errorprone compiler plugin in build
• for now, errorprone errors are converted to warnings with the -XepAllErrorsAsWarnings flag

[codecov-io]

Codecov Report

Merging #349 into master will decrease coverage by 0.4%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #349      +/-   ##
============================================
- Coverage     69.35%   68.94%   -0.41%     
+ Complexity     1051     1045       -6     
============================================
  Files            72       72              
  Lines          4386     4399      +13     
  Branches        628      629       +1     
============================================
- Hits           3042     3033       -9     
- Misses          964      983      +19     
- Partials        380      383       +3

Impacted Files	Coverage Δ	Complexity Δ
.../ipc/netty/channel/PooledClientContextHandler.java	58.82% <0%> (-9.25%)	24% <0%> (-3%)
...ctor/ipc/netty/resources/DefaultPoolResources.java	75.86% <0%> (-3.45%)	8% <0%> (-1%)
...ava/reactor/ipc/netty/resources/LoopResources.java	30% <0%> (-3.34%)	9% <0%> (ø)
src/main/java/reactor/ipc/netty/FutureMono.java	67.5% <0%> (-1.67%)	8% <0%> (ø)
...actor/ipc/netty/http/server/HttpServerHandler.java	77.93% <0%> (-1.09%)	41% <0%> (-1%)
src/main/java/reactor/ipc/netty/udp/UdpClient.java	76.47% <0%> (-0.68%)	10% <0%> (ø)
src/main/java/reactor/ipc/netty/udp/UdpServer.java	76.47% <0%> (-0.68%)	10% <0%> (ø)
...or/ipc/netty/http/server/HttpServerOperations.java	67.19% <0%> (-0.53%)	53% <0%> (-1%)
...n/java/reactor/ipc/netty/options/NettyOptions.java	72.8% <0%> (ø)	24% <0%> (ø)	⬇️
src/main/java/reactor/ipc/netty/tcp/TcpClient.java	90.56% <0%> (ø)	20% <0%> (ø)	⬇️
... and 6 more

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[violetagg]

#350 will track the fixes for the warnings produced by this plugin