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
