Add hash and direct pick strategies

[sevenhe1205]

Add hash and direct pick strategies

The grpcbox_client get_channel function adds support for hash and direct strategies gproc_pool:pick_worker uses the hash/direct policy. Options do not contain the Key parameter. gproc_pool:pick_worker uses the round_robin/random strategy

[tsloughter]

The reason I didn't have these is they aren't in the grpc LB spec https://github.com/grpc/grpc/blob/master/doc/load-balancing.md#load-balancing-policies

But maybe that shouldn't stop us...