soroban-rpc: Graceful shutdown for HTTP server

[paulbellamy]

What problem does your feature solve?

When you kill soroban-rpc it may drop ongoing HTTP requests.

What would you like to see?

We should use the HTTP server graceful shutdown mechanism to finish pending HTTP requests before shutdown.

What alternatives are there?

[2opremio]

We are already using gopkg.in/tylerb/graceful.v1 (through supporthttp.Run())

[2opremio]

Although we should probably just be using https://pkg.go.dev/net/http#Server.Shutdown directly