lnd: implement open+close channel workflow in daemon

This commit adds the necessary plumbing within the server, peer, and
rpcServer to handle opening and cooperatively closing a channel with a
remote peer.

Many new data structures have been added to the peer in order to allow
it to efficiently manage opening+.losing new/existing lightning
channels. Additional documentation has been added to several methods
within the peer struct, with some minor renaming along with way. The
peer has also gained a dedicated goroutine whose job it is to manage
any requests pertaining to opening, or closing any channels with the
remote peer.

The messages have been added to lnrpc define the requests and responses
to channel open+close messages. Additional channel logic has been added
between the rpcServer, peer, and server in order to properly manage the
necessary synchronization.

lnrpc/rpc.proto

@@ -4,6 +4,7 @@ package lnrpc;
 
 service Lightning {
     rpc SendMany(SendManyRequest) returns (SendManyResponse);
+
     rpc NewAddress(NewAddressRequest) returns (NewAddressResponse);
 
     rpc ConnectPeer(ConnectPeerRequest) returns (ConnectPeerResponse);
@@ -97,6 +98,31 @@ message ListPeersRequest {}
 message ListPeersResponse {
     repeated Peer peers = 1;
 }
+
+message OpenChannelRequest {
+    int32 target_peer_id = 1;
+    LightningAddress target_node = 2;
+
+    int64 local_funding_amount = 3;
+    int64 remote_funding_amount = 4;
+
+    int64 commission_size = 5;
+
+    uint32 num_confs = 6;
+}
+message OpenChannelResponse {
+    ChannelPoint channel_point = 1;
+}
+
+message CloseChannelRequest {
+    ChannelPoint channel_point = 1;
+    int64 time_limit = 2;
+    bool allow_force_close = 3;
+}
+message CloseChannelResponse {
+    bool success = 1;
+}
+
 message WalletBalanceRequest {
     bool witness_only = 1;
 }