I2CP Multisession support and multiple destinations in one tunnel pool.

Work in progress.
Router-side I2CP mostly done.
Client-side I2CP mostly done but undecided on how to handle
listeners.
Streaming stubbed out but may be wrong, may need multiple socket managers,
not clear how to proceed.
I2PTunnel not started.
Blacklist of DSA-only dests not started.
Router leaseset publishing not correct. Not clear whether to have
additional tunnel pools with flags, or put the tunnel pools into
the client hashmap twice. Client config contains destination,
may need to move that to tunnel pool.