[group key addrs 1/?]: add new authmailbox RPC server and client

[guggero]

Implements the mailbox server and client as described here: #874 (comment)

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 15734766059

Details

• 818 of 1232 (66.4%) changed or added relevant lines in 12 files are covered.
• 41 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.5%) to 37.798%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
authmailbox/message.go	3	6	50.0%
internal/test/grpc.go	8	12	66.67%
authmailbox/stream.go	20	26	76.92%
authmailbox/mock.go	41	56	73.21%
internal/test/mock.go	17	55	30.91%
proof/tx.go	106	144	73.61%
authmailbox/client.go	81	127	63.78%
authmailbox/receive_subscription.go	207	271	76.38%
proof/mock.go	0	85	0.0%
authmailbox/server.go	321	436	73.62%

Files with Coverage Reduction	New Missed Lines	%
asset/mock.go	2	65.45%
tappsbt/create.go	2	26.74%
tapchannel/aux_leaf_signer.go	3	43.43%
tapchannel/aux_invoice_manager.go	6	80.51%
tapgarden/caretaker.go	6	68.68%
address/address.go	8	67.47%
address/mock.go	14	88.24%

Totals
Change from base Build 15712916525:	0.5%
Covered Lines:	28115
Relevant Lines:	74383

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💛 - Coveralls

[Roasbeef]

Reviewed 32 of 32 files at r1, all commit messages.
Reviewable status: all files reviewed, 10 unresolved discussions (waiting on @ffranr and @guggero)

[guggero]

I removed the sender's ephemeral key from the proto. It will be part of the encrypted payload, passed into the EncryptSha256ChaCha20Poly1305 function as additionalData.

[lightninglabs-deploy]

@Roasbeef: review reminder
@ffranr: review reminder

[Roasbeef]

LGTM 🦋

Reviewed 12 of 12 files at r2, all commit messages.
Reviewable status: all files reviewed, 19 unresolved discussions (waiting on @ffranr and @guggero)

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authmailbox/client.go line 133 at r2 (raw file):

func (c *Client) Stop() error {
	var stopErr error
	c.stopOnce.Do(func() {

👍 for sync.Once.

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authmailbox/server.go line 99 at r2 (raw file):

// Start signals that the RPC server starts accepting requests.
func (s *Server) Start() error {
	if !s.started.CompareAndSwap(false, true) {

sync.Once here again to be consistent with the rest of the PR?