Failed transactions cause a memory leak

[mtusnio]

Any failed COAP transactions that get sent with Erbium are added to a list of transactions, however they are never removed. From my experience this is because coap_clear_transaction is never called here: https://github.com/FlowM2M/AwaLWM2M/blob/master/core/src/erbium/er-coap-transactions.c#L98
After a while the malloc in coap_new_transaction start returning NULL due to lack of memory.

However, adding coap_clear_transaction there causes any successful requests to fail since they also go into that branch of the if, so it doesn't seem like it's just an easy fix of adding that in.

[boyvinall]

The transactions should be cleared after MAX_TRANSMIT_SPAN, see http://www.rfcreader.com/#rfc7252_line1260