Write a load generator binary

[dinowernli]

This should be a standalone subcommand of the almanac binary. It should be possible to point the binary at an ingester and it should fire rpcs at the ingester to generate load. Various parameters should be controllable by flags, e.g., log size, log frequency, etc.

./almanac loadgen --log_size_bytes=1000 --logs_per_second=5 --ingesters=localhost:123,localhost:124