Upgrade React/Http to v0.8

[andig]

Supplements php-pm/php-pm#271

• wait for Cannot use HttpServer with UDS reactphp/http#231 (replaced by custom UnixServer)
• wait for RequestBodyBufferMiddleware: 411 No Content Length for GET requests? reactphp/http#232 (nice to have but not required)
• testing (@marcj)
• remove proxy repos

Points for follow-up:

• remove minimum stability when react/http 0.8 has been released

[dzubchik]

@andig , I would like to invest some time into stabilization http-kernel adapter and cover it with tests. Do you think it is right time for it? I plan to write tests for next things:

• stateless requests
• statefull requests
• file uplodaing
• file serving
• etc

[andig]

@dzubchik sounds great! Two points for consideration:

• ppm already has some tests that use the kernel adapter- might make sense to start with these and not duplicate
• I'd personally suggest to base new tests on this PR/ PSR7 as basis although it's not merged yet, so opinions might differ