Add parsing a directory to get all certificates in it

optimize belle_sip_object_t (size field moved to vptr)
fix Makefile warning.

when pending transactions are queued in the dialog, newly created transaction must be automatically queued as well, in order to preserve request order.
Previous commit broke the normal way of sending a request through a transaction by not honoring the "outbound proxy" argument.

This reverts commit 8b1a7977.

expected.)

The object-oriented architecture was not used correctly for the TLS channel. The call to belle_sip_channel_set_socket was reinitializing the belle_sip_source_t and so erasing the timeout value initialized by stream_channel_connect.
The fix is to just replace the notify callback (that was the only goal of the call to belle_sip_channel_set_socket).
Also added a unit test to check this.

Use specific background task for HTTP transfers, otherwise the channel might hang when an iOS app goes into background

* implement retry of EWOULDBLOCK returned send()
* rework channel's listener for clarity
* fix various memory leaks.

progress in http implementation

I.E just a /

- supports IPv6 DNS servers
- supports nameserver lists with both IPv4 and IPv6 servers

Added a belle_sip_dict_t that has base accessors over this storage, and the appropriate unit tests.