fix gnutls download (closes #122)

Please merge this fix,

Regards,

Frédéric

See merge request !38

This reverts commit 3f20ce7b.

it was mistakenly pushed to develop

thanks to Supreeth Herle for reporting.

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This was causing oaisim to fail way too often at the startup
of a connection.

Things should go better with this patch.

It may not be the end of the story, the use of retrans_num_bytes_to_retransmit
is not clear to me. The problem I see is: what if the max number of
retransimission has been reached (if that has a meaning at all in RLC AM)?
will this value be decreased or not?

The code was changing the rnti in the red-black tree without
removing/reinserting the context. This is wrong because the
rnti is a key of the red-black tree and you cannot change the
key in a stored item at will.

There are also modifications of the function mac_eNB_rrc_ul_failure
which has to remove the UE in any case.

And rrc_mac_remove_ue has been changed for the case where
the UE is not found.