[core] fix double memory free crash in observation

[GiedriusM]

• lwm2m_observe_cancel instead of sending unobserve packet, now changes observation state
• lwm2m_observe_cancel calls callback immediately
• Number of pending transactions is tracked to prevent premature free

[GiedriusM]

@travis-ci

[codecov[bot]]

Codecov Report

Merging #74 into master-rest will not change coverage. The diff coverage is n/a.

@@             Coverage Diff              @@
##           master-rest      #74   +/-   ##
============================================
  Coverage        80.83%   80.83%           
============================================
  Files               13       13           
  Lines             1080     1080           
============================================
  Hits               873      873           
  Misses             207      207

Flag	Coverage Δ
#regular	74.25% <ø> (ø)	:arrow_up:
#secure	25.64% <ø> (ø)	:arrow_up:

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