Set libmicrohttpd connections limit in http transport configuration.

[atoppi]

This PR lets the user define the maximum number of open connection in libmicrohttpd through the mhd_connection_limit setting in the http transport configuration.

As explained in the official docs.

MHD_OPTION_CONNECTION_LIMIT
Maximum number of concurrent connections to accept (followed by an unsigned int). The default is FD_SETSIZE - 4

(FD_SETSIZE is 1024 in most environments)
This should be useful for deployments that make intensive use of http transport and could hit that limit.
Default value has been preserved to FD_SETSIZE - 4.

[lminiero]

Merging 👍