REORG: connection: move the largest inlines from connection.h to connection.c

REORG: connection: move the largest inlines from connection.h to connection.c

The following inlined functions are particularly large (and probably not
inlined at all by the compiler), and together represent roughly half of
the file, while they're used at most once per connection. They were moved
to connection.c.

  conn_upgrade_mux_fe, conn_install_mux_fe, conn_install_mux_be,
  conn_install_mux_chk, conn_delete_from_tree, conn_init, conn_new,
  conn_free