Add workaround to update_sapling_tree for a lightwalletd behind a load balancer

[str4d]

The update_sapling_tree binary uses the grpc crate, which in turn uses the httpbis crate to make connections. The currently hard-coded host is lightwalletd.z.cash, which has (I assume) recently been configured to use a load balancer instead of a single IP address. However, httpbis cannot handle a domain name that resolves to multiple IP addresses, and returns an error (stepancheg/rust-http2#7).

The workaround is to perform DNS resolution ourselves, select one of the IPs, and then use that IP specifically.