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).

50aa58a7d47d168ec550762c25d7aaaed3f4177b   ; oid=0..16995  nread=68269354  t=638.903389ms (37.591µs / object)  x=zsha1.go

No big speedup measurable but very visible in tracing.

before:

BenchmarkLinkNetPipeRTT-4         200000              9763 ns/op            9664 B/op         25 allocs/op
BenchmarkLinkTCPRTT-4             100000             19069 ns/op            9664 B/op         25 allocs/op

after:

BenchmarkLinkNetPipeRTT-4         200000              7872 ns/op            1224 B/op         17 allocs/op
BenchmarkLinkTCPRTT-4             100000             18465 ns/op            1221 B/op         17 allocs/op