pss: fix concurrent map read and map write on pubKeyRWPool

[janos]

This PR adds read protection to pool maps in pss handler. Map is protected on write operations, but not on read resulting the following panic:

WARN [10-04|12:12:29.393|github.com/ethersphere/swarm/pss/crypto/crypto.go:183] Size of payload field                    size=1
WARN [10-04|12:12:29.395|github.com/ethersphere/swarm/pss/crypto/crypto.go:183] Size of payload field                    size=1
WARN [10-04|12:12:29.398|github.com/ethersphere/swarm/pss/crypto/crypto.go:183] Size of payload field                    size=1
WARN [10-04|12:12:29.399|github.com/ethersphere/swarm/pss/crypto/crypto.go:183] Size of payload field                    size=1
fatal error: concurrent map read and map write

goroutine 164 [running]:
runtime.throw(0xdbbf54, 0x21)
    /usr/local/go/src/runtime/panic.go:774 +0x72 fp=0xc0013678a0 sp=0xc001367870 pc=0x435ed2
runtime.mapaccess1_faststr(0xc6cd40, 0xc000589830, 0xc0005acfc0, 0x84, 0x3)
    /usr/local/go/src/runtime/map_faststr.go:21 +0x44f fp=0xc001367910 sp=0xc0013678a0 pc=0x41922f
github.com/ethersphere/swarm/pss.(*Protocol).Handle(0xc000585700, 0xc0013ae5a2, 0x8, 0x59, 0xc000573140, 0xc000573101, 0xc0005acfc0, 0x84, 0xc000573140, 0xc0000e5180)
    /home/rafa/go/src/github.com/ethersphere/swarm/pss/protocol.go:187 +0x1a2 fp=0xc0013679c0 sp=0xc001367910 pc=0xb513c2
github.com/ethersphere/swarm/pss.(*Protocol).Handle-fm(0xc0013ae5a2, 0x8, 0x59, 0xc000573140, 0xc0013c8001, 0xc0005acfc0, 0x84, 0x0, 0x0)
    /home/rafa/go/src/github.com/ethersphere/swarm/pss/protocol.go:166 +0x7c fp=0xc001367a20 sp=0xc0013679c0 pc=0xb82b9c
github.com/ethersphere/swarm/pss.(*Pss).executeHandlers(0xc000266000, 0xc04e61647f, 0xc0013ae5a2, 0x8, 0x59, 0xc000570160, 0x20, 0x20, 0x10000, 0xc0005acfc0, ...)
    /home/rafa/go/src/github.com/ethersphere/swarm/pss/pss.go:614 +0x29e fp=0xc001367b28 sp=0xc001367a20 pc=0xb56b9e
github.com/ethersphere/swarm/pss.(*Pss).process(0xc000266000, 0xc0013b22c0, 0xc001350000, 0x0, 0x0)
    /home/rafa/go/src/github.com/ethersphere/swarm/pss/pss.go:586 +0x23c fp=0xc001367c68 sp=0xc001367b28 pc=0xb564fc
github.com/ethersphere/swarm/pss.(*Pss).handlePssMsg(0xc000266000, 0xf17860, 0xc0000b0010, 0xc0013b22c0, 0x0, 0x0)
    /home/rafa/go/src/github.com/ethersphere/swarm/pss/pss.go:543 +0x662 fp=0xc001367f20 sp=0xc001367c68 pc=0xb557e2
github.com/ethersphere/swarm/pss.(*Pss).handle.func1(0xc89ee0, 0xc0013b22c0, 0xc00053c0f0, 0xc000266000, 0xf17860, 0xc0000b0010)
    /home/rafa/go/src/github.com/ethersphere/swarm/pss/pss.go:487 +0x85 fp=0xc001367fb0 sp=0xc001367f20 pc=0xb7c125
runtime.goexit()
    /usr/local/go/src/runtime/asm_amd64.s:1357 +0x1 fp=0xc001367fb8 sp=0xc001367fb0 pc=0x4660b1
created by github.com/ethersphere/swarm/pss.(*Pss).handle
    /home/rafa/go/src/github.com/ethersphere/swarm/pss/pss.go:481 +0x71

[...]

goroutine 112 [select]:
github.com/ethersphere/swarm/pss.NewPingProtocol.func1.1(0xc0005b4960, 0xc00058c660, 0xc0000953e0)
    /home/rafa/go/src/github.com/ethersphere/swarm/pss/ping.go:82 +0xb4
created by github.com/ethersphere/swarm/pss.NewPingProtocol.func1
    /home/rafa/go/src/github.com/ethersphere/swarm/pss/ping.go:80 +0x1f4

FAIL    github.com/ethersphere/swarm/pss    1.166s

This panic was reported by @mortelli https://beehive.ethswarm.org/swarm/pl/butadain33bjzyasrkqoyhtdah, but I could not reproduce it.

[kortatu]

LGTM