Instead of providing our destination (private key) to the I2P proxy when
creating the session, ask it to generate one for us and do not save it
on disk.
Change the types of `i2p::Connection::sock` and
`i2p::sam::Session::m_control_sock` from `Sock` to
`std::unique_ptr<Sock>`.
Using pointers would allow us to sneak `FuzzedSock` instead of `Sock`
and have the methods of the former called.
After this change a test only needs to replace `CreateSock()` with
a function that returns `FuzzedSock`.
Put a limit on the amount of data `Sock::RecvUntilTerminator()` can read
if no terminator is received.
In the case of I2P this avoids a runaway (or malicious) I2P proxy
sending us tons of data without a terminator before a timeout is
triggered.
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
