Update mbed-coap to version 5.1.6

[esajaa]

Summary of changes

Multiple fixes for out-ouf-bounds memory accesses, a memory leak and an infinite loop condition in packet parser.

Mapped MBED_CONF_MBED_CLIENT_RECONNECTION_COUNT Pelion Device Management Client configuration option to SN_COAP_RESENDING_MAX_COUNT in mbed-coap.

Fix the token from blockwise ACK message to be empty if the received message doesn't have one.

This fixes following Github issues:
#12957
#12948
#12930
#12928
#12927
#12926
#12925

Impact of changes

Migration actions required

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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@yogpan01 @ristohuhtala

[yogpan01]

Approved.
@0xc0170 @adbridge can you please merge this. It fixes issues reported in mbedOS.

[ciarmcom]

@esajaa, thank you for your changes.
@ristohuhtala @yogpan01 @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

It will be in CI later today (merged as well) once we complete 6.1 release testing.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts