With some care it is possible for serveRecv to put itself to pause and
handoff execution to goroutine for which a packet arrived.

See comments in serveRecv about how it is done.

For BenchmarkTCPlosr cuts RTT 12.5μs -> 6.6μs .

BenchmarkSyncChanRTT-4           5000000               350 ns/op
BenchmarkBufChanRTT-4            5000000               352 ns/op
BenchmarkBufChanAXRXRTT-4        3000000               407 ns/op
BenchmarkNetPipeRTT-4            2000000               938 ns/op
BenchmarkNetPipeRTTsr-4          1000000              1594 ns/op	<-- here
BenchmarkTCPlo-4                  300000              4814 ns/op
BenchmarkTCPlosr-4                100000             12261 ns/op	<-- here
BenchmarkLinkNetPipeRTT-4         500000              3017 ns/op
BenchmarkLinkTCPRTT-4             100000             15650 ns/op

the δ beetwen TCPlo + serveRecv style RX and full link over TCPlo is ~ 3μs.

-> need to find out why TCPlosr = TCPlo + 8μs

without runtime.Gosched

null:00   ; oid=0..16995  nread=68269354  t=536.560158ms (31.569µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=532.416867ms (31.326µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=536.958977ms (31.593µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=534.170594ms (31.429µs / object)  x=zsha1.go

with runtime.Gosched

null:00   ; oid=0..16995  nread=68269354  t=594.966346ms (35.006µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=597.510359ms (35.155µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=598.251026ms (35.199µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=596.02138ms (35.068µs / object)  x=zsha1.go

----

-> trace shows runtime.Gosched indeed switches to woken up G without
second syscall in serveRecv but serveRecv migrates to different M (so
different CPU)

LinkNetPipeRTT-4  3.05µs ± 1%  2.99µs ± 0%   -2.05%  (p=0.008 n=5+5)
LinkTCPRTT-4      15.9µs ± 1%  14.3µs ± 2%  -10.11%  (p=0.008 n=5+5)

- BenchmarkLinkNetPipeRTT-4         500000              3189 ns/op             225 B/op          5 allocs/op
+ BenchmarkLinkNetPipeRTT-4         500000              3035 ns/op             225 B/op          5 allocs/op

- BenchmarkLinkNetPipeRTT-4         500000              3555 ns/op             225 B/op          5 allocs/op
+ BenchmarkLinkNetPipeRTT-4         500000              3189 ns/op             225 B/op          5 allocs/op

- BenchmarkLinkNetPipeRTT-4         500000              3768 ns/op             225 B/op          5 allocs/op
+ BenchmarkLinkNetPipeRTT-4         500000              3555 ns/op             225 B/op          5 allocs/op

- BenchmarkLinkNetPipeRTT-4         300000              4825 ns/op             225 B/op          5 allocs/op
+ BenchmarkLinkNetPipeRTT-4         500000              3807 ns/op             225 B/op          5 allocs/op

- BenchmarkLinkNetPipeRTT-4         300000              5668 ns/op             225 B/op          5 allocs/op
+ BenchmarkLinkNetPipeRTT-4         300000              4825 ns/op             225 B/op          5 allocs/op

automatically removes select serveRecv.

null:00   ; oid=0..16995  nread=68269354  t=815.489603ms (47.981µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=809.428095ms (47.624µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=815.088024ms (47.957µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=803.725121ms (47.289µs / object)  x=zsha1.go

before:

null:00   ; oid=0..16995  nread=68269354  t=481.582632ms (28.335µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=473.499859ms (27.859µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=471.996668ms (27.771µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=478.029272ms (28.125µs / object)  x=zsha1.go

after:

null:00   ; oid=0..16995  nread=68269354  t=709.761334ms (41.76µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=704.768088ms (41.466µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=720.756186ms (42.407µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=693.688744ms (40.814µs / object)  x=zsha1.go

now we'll be teaching Recv1 & friends to do things in optimized way but Conn
functionality must stay working.

if rxq is buffered it adds only ~50ns to SyncChanRTT (was ~ 350ns ->
400ns) and important: no extra goroutines switches are introduced
(verified via analyzing trace).