go/p2p/rpc: Refactor rpc calls

[peternose]

Peer selection is very tightly coupled with RPC client as peer manager is constructed inside client's constructor. We also miss simple RPC calls which contact exactly one peer.

Test

Memory leak was check with the following code, which prints 1 and 101, meaning that 100 go routines leaked.

package main

import (
	"fmt"
	"runtime"
	"time"

	"github.com/eapache/channels"
)

func main() {
	fmt.Println(runtime.NumGoroutine())
	for i := 0; i < 100; i++ {
		leak()
	}
	time.Sleep(time.Minute)
	fmt.Println(runtime.NumGoroutine())
}

func leak() {
	n := 10
	var chs []channels.SimpleOutChannel

	for i := 0; i < n; i++ {
		ch := channels.NewNativeChannel(channels.BufferCap(1))
		defer close(ch)

		chs = append(chs, ch)
		ch <- i
	}

	ch := channels.NewNativeChannel(channels.None)
	channels.Multiplex(ch, chs...)

	for v := range ch.Out() {
		if v.(int) < n/2 {
			break
		}
	}
}

[codecov]

Codecov Report

Merging #5007 (b548870) into master (a888835) will decrease coverage by 0.06%.
The diff coverage is 89.69%.

@@            Coverage Diff             @@
##           master    #5007      +/-   ##
==========================================
- Coverage   66.68%   66.62%   -0.07%     
==========================================
  Files         478      478              
  Lines       51800    51868      +68     
==========================================
+ Hits        34544    34556      +12     
- Misses      12983    13041      +58     
+ Partials     4273     4271       -2     

Impacted Files	Coverage Δ
go/p2p/rpc/server.go	78.72% <ø> (ø)
go/p2p/rpc/nop.go	6.66% <20.00%> (+6.66%)	⬆️
go/p2p/rpc/client.go	79.73% <87.38%> (+0.11%)	⬆️
go/p2p/rpc/peermgmt.go	82.35% <93.75%> (+2.97%)	⬆️
go/p2p/txsync/client.go	72.97% <100.00%> (+4.22%)	⬆️
go/worker/keymanager/p2p/client.go	81.17% <100.00%> (+0.22%)	⬆️
go/worker/storage/p2p/pub/client.go	84.61% <100.00%> (+3.66%)	⬆️
go/worker/storage/p2p/sync/client.go	89.28% <100.00%> (-2.02%)	⬇️
go/roothash/api/sanity_check.go	40.00% <0.00%> (-20.00%)	⬇️
go/worker/storage/committee/utils.go	92.30% <0.00%> (-7.70%)	⬇️
... and 37 more

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[kostko]

Can you also backport the memory leak and peer scoring fixes?