Make max number of HTTP/2 streams configurable

Make max number of HTTP/2 streams configurable

This commit makes number of concurrent HTTP/2 streams per connection
configurable as a mean to fight DDoS attacks. As soon as the limit is
reached, BIND terminates the whole session.

The commit adds a global configuration
option (http-streams-per-connection) which can be overridden in an
http <name> {...} statement like follows:

http local-http-server {
    ...
    streams-per-connection 100;
    ...
};

For now the default value is 100, which should be enough (e.g. NGINX
uses 128, but it is a full-featured WEB-server). When using lower
numbers (e.g. ~70), it is possible to hit the limit with
e.g. flamethrower.