Rewrite SocketPlugin to NIO

[f4lco]

This PR

• splits TCP and UDP sockets in separate classes
• uses non-blocking I/O primitives throughout
• (reliably) greens dozens of test selectors from SocketTest, SocketStreamTest, SqueakSSLTest, StringSocketTestCase, WebClientServerTest, ...
• implements features that were missing from the previous implementation: getting / setting socket options, respecting different TCP backlog sizes, and "in-place accept" (such that one can put the socket in listen mode, and call read/write on the same instance, which are then relayed to the last accepted client connection)

[fniephaus]

Cool! Thanks a lot, will have a look soon!

[codecov]

Codecov Report

Merging #21 into dev will increase coverage by 0.65%.
The diff coverage is 65.85%.

@@             Coverage Diff              @@
##                dev      #21      +/-   ##
============================================
+ Coverage     52.13%   52.78%   +0.65%     
- Complexity     2454     2525      +71     
============================================
  Files           189      193       +4     
  Lines         17061    17069       +8     
  Branches       2403     2362      -41     
============================================
+ Hits           8894     9010     +116     
+ Misses         7231     7134      -97     
+ Partials        936      925      -11

Impacted Files	Coverage Δ	Complexity Δ
...a/graal/squeak/exceptions/PrimitiveExceptions.java	73.33% <ø> (ø)	0 <0> (ø)	⬇️
.../squeak/nodes/primitives/PrimitiveNodeFactory.java	75.8% <ø> (ø)	20 <0> (ø)	⬇️
...l/squeak/test/AbstractSqueakTestCaseWithImage.java	73.45% <100%> (+0.47%)	28 <0> (ø)	⬇️
...aal/squeak/nodes/plugins/network/SocketPlugin.java	53.5% <53.5%> (ø)	4 <4> (?)
.../squeak/nodes/plugins/network/SqueakUDPSocket.java	65.85% <65.85%> (ø)	11 <11> (?)
...aal/squeak/nodes/plugins/network/SqueakSocket.java	69.87% <69.87%> (ø)	14 <14> (?)
...a/graal/squeak/nodes/plugins/network/Resolver.java	76.47% <76.47%> (ø)	9 <9> (?)
.../squeak/nodes/plugins/network/SqueakTCPSocket.java	84.61% <84.61%> (ø)	35 <35> (?)
...nodes/primitives/impl/MiscellaneousPrimitives.java	59.51% <0%> (-0.35%)	2% <0%> (ø)
... and 7 more

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[fniephaus]

LGTM