Remove transaction only when max retries are reached

[Lycopersicum]

I removed transacP->ack_received checking, because if observe cancellation is requested, sometimes two messages are detected, calling free() two times (which causes segmentation fault). To reproduce this problem:

1. Connect sensor(-s) to restserver.
2. PUT request to /subscriptions/<connected-sensor-UUID>/<path-to-resource> (must be a valid, observable resource).
3. DELETE request to /subscriptions/<connected-sensor-UUID>/<path-to-resource> (observation must be started already).

If problem doesn't occur, repeat last two steps until it does.

Note: You can reproduce the problem only if observation removal is implemented.

[codecov-io]

Codecov Report

Merging #16 into master-rest will decrease coverage by 33.34%. The diff coverage is 100%.

@@               Coverage Diff                @@
##           master-rest      #16       +/-   ##
================================================
- Coverage        65.04%   31.69%   -33.35%     
================================================
  Files                9       26       +17     
  Lines              615     4010     +3395     
================================================
+ Hits               400     1271      +871     
- Misses             215     2739     +2524

Impacted Files	Coverage Δ
core/transaction.c	67.11% <100%> (ø)
core/json.c	0% <0%> (ø)
core/uri.c	45.45% <0%> (ø)
core/data.c	0% <0%> (ø)
core/observe.c	45.25% <0%> (ø)
core/block1.c	0% <0%> (ø)
core/management.c	19.31% <0%> (ø)
examples/shared/commandline.c	0% <0%> (ø)
core/packet.c	65.93% <0%> (ø)
... and 8 more

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