release v3.1.0

[sreimers]

ABI bump is not needed, since only added functions:

Functions changes summary: 0 Removed, 0 Changed, 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable
Function symbols changes summary: 0 Removed, 12 Added function symbols not referenced by debug info
Variable symbols changes summary: 0 Removed, 0 Added variable symbol not referenced by debug info

12 Added function symbols not referenced by debug info:

  [A] http_sock_tls
  [A] https_set_verify_msgh
  [A] tls_set_posthandshake_auth
  [A] tls_set_verify_client_handler
  [A] tls_set_verify_client_trust_all
  [A] tls_verify_client_post_handshake
  [A] tmr_continue_dbg
  [A] tmr_timespec_get
  [A] uri_display_name_escape
  [A] uri_escape
  [A] uri_escape_pl
  [A] vidmix_source_get_pidx