net: implement the necessary parts of the I2P SAM protocol

Implement the following commands from the I2P SAM protocol:

* HELLO: needed for all of the remaining ones
* DEST GENERATE: to generate our private key and destination
* NAMING LOOKUP: to convert .i2p addresses to destinations
* SESSION CREATE: needed for STREAM CONNECT and STREAM ACCEPT
* STREAM CONNECT: to make outgoing connections
* STREAM ACCEPT: to accept incoming connections

src/logging.h

@@ -57,6 +57,7 @@ namespace BCLog {
         QT          = (1 << 19),
         LEVELDB     = (1 << 20),
         VALIDATION  = (1 << 21),
+        I2P         = (1 << 22),
         ALL         = ~(uint32_t)0,
     };