Replace the home-made http subroutines with LWP in the client

[oyvindhagberg]

• nivlheim_client should use LWP instead of the homemade subroutines http_get, http_post and ssl_connect. Remove these.

❗ This has to wait until 30.nov 2020, when RHEL6 has reached end-of-life, as it is prohibitively hard to make IPv6 work with the SSL Perl libraries on RHEL6.