play: Enable TLS SNI for Dgraph Play queries

[danielmai]

This sets proxy_ssl_server_name so that the SNI is used in the TLS proxy connection.

Without this, nginx won't be able to connect to the upstream backends. These logs show up in error.log:

2021/05/04 15:48:55 [error] 22269#22269: *42 no live upstreams while connecting to upstream, client: 172.69.79.178, server: play.dgraph.io, request: "GET /health HTTP/1.1", upstream: "https://crooked-advertisement.us-west-2.aws.cloud.dgraph.io/health", host: "play.dgraph.io", referrer: "https://play.dgraph.io/?latest"

<...nginx reload...>

2021/05/04 15:51:10 [error] 23517#23517: *120 SSL_do_handshake() failed (SSL: error:14094438:SSL routines:ssl3_read_bytes:tlsv1 alert internal error:SSL alert number 80) while SSL handshaking to upstream, client: $IP, server: play.dgraph.io, request: "POST /query?timeout=20s HTTP/1.1", upstream: "https://$IP:443/query?timeout=20s&be=true", host: "play.dgraph.io", referrer: "https://play.dgraph.io/?latest"

This fixes connection issues for queries running in:

• the Dgraph docs (e.g., https://dgraph.io/docs/query-language/functions), and
• the GraphQL Tour (e.g., https://dgraph.io/tour/graphqlmoredata/2/).

This PR also bumps up the limit requests on Play so that more traffic can get through.

References:

• https://stackoverflow.com/a/38401715
• http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ssl_server_name

This change is 

[danielmai]

I'm not sure why proxy_ssl_server_name has only now started to be set, but it does address the issues in the error log.

Reviewable status: 0 of 1 files reviewed, all discussions resolved

[roshbhatia]

lgtm