implement database and mqtt client to cache node info

src/protos/meshtastic/xmodem.proto

@@ -0,0 +1,27 @@
+syntax = "proto3";
+
+package meshtastic;
+
+option csharp_namespace = "Meshtastic.Protobufs";
+option go_package = "github.com/meshtastic/go/generated";
+option java_outer_classname = "XmodemProtos";
+option java_package = "com.geeksville.mesh";
+option swift_prefix = "";
+
+message XModem {
+  enum Control {
+    NUL = 0;
+    SOH = 1;
+    STX = 2;
+    EOT = 4;
+    ACK = 6;
+    NAK = 21;
+    CAN = 24;
+    CTRLZ = 26;
+  }
+
+  Control control = 1;
+  uint32 seq = 2;
+  uint32 crc16 = 3;
+  bytes buffer = 4;
+}