restserver: fix memory leaks

[GiedriusM]

• JSON object leak in notifications channel
• CoAP transaction payload leak in resource write and execute
• Rest and lwm2m context cleanup on exit (sugar for valgrind)

[codecov-io]

Codecov Report

Merging #21 into master-rest will increase coverage by 1.52%. The diff coverage is 80.55%.

@@               Coverage Diff               @@
##           master-rest      #21      +/-   ##
===============================================
+ Coverage        65.04%   66.56%   +1.52%     
===============================================
  Files                9        9              
  Lines              615      637      +22     
===============================================
+ Hits               400      424      +24     
+ Misses             215      213       -2

Impacted Files	Coverage Δ
examples/rest-server/rest-list.c	85.36% <100%> (+4.72%)	:arrow_up:
examples/rest-server/rest-core.c	57.14% <100%> (+12.69%)	:arrow_up:
examples/rest-server/restserver.c	73.21% <100%> (+0.48%)	:arrow_up:
examples/rest-server/rest-resources.c	49.5% <50%> (+3.04%)	:arrow_up:
examples/rest-server/rest-notifications.c	55.83% <75%> (ø)	:arrow_up:

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