feature:http2 support multiplex and keepalive

[duxin40]

feature:http2 support multiplex and keepalive.

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[codecov]

Codecov Report

Base: 60.35% // Head: 59.85% // Decreases project coverage by -0.50% ⚠️

Coverage data is based on head (d398b5b) compared to base (f39f9e1).
Patch coverage: 1.42% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2165      +/-   ##
==========================================
- Coverage   60.35%   59.85%   -0.51%     
==========================================
  Files         417      421       +4     
  Lines       36583    36933     +350     
==========================================
+ Hits        22081    22106      +25     
- Misses      12319    12646     +327     
+ Partials     2183     2181       -2     

Impacted Files	Coverage Δ
pkg/stream/http2/connpool.go	0.00% <0.00%> (ø)
pkg/stream/http2/connpool_multiplex.go	0.00% <0.00%> (ø)
pkg/stream/http2/idlefree.go	0.00% <0.00%> (ø)
pkg/stream/http2/keepalive.go	0.00% <0.00%> (ø)
pkg/stream/http2/stream.go	38.13% <7.69%> (-0.67%)	⬇️
pkg/stream/http2/keepalive_configure.go	100.00% <100.00%> (ø)
pkg/stream/stream.go	33.33% <0.00%> (-26.67%)	⬇️
...ream/faulttolerance/regulator/default_work_pool.go	88.63% <0.00%> (-4.55%)	⬇️
pkg/upstream/cluster/loadbalancer.go	73.70% <0.00%> (-0.80%)	⬇️
pkg/server/handler.go	39.78% <0.00%> (-0.54%)	⬇️
... and 6 more

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