boost 1.70 compatibility

[RvdE]

Fixes #1880

[codecov-io]

Codecov Report

Merging #1881 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1881      +/-   ##
==========================================
+ Coverage   90.11%   90.11%   +<.01%     
==========================================
  Files         526      526              
  Lines       55936    55936              
  Branches     5825     5825              
==========================================
+ Hits        50405    50406       +1     
+ Misses       5531     5530       -1

Impacted Files	Coverage Δ
src/cli/tls_proxy.cpp	4.97% <0%> (ø)	⬆️
src/cli/tls_http_server.cpp	4.24% <0%> (ø)	⬆️
src/lib/tls/tls_channel.cpp	80.19% <0%> (-0.34%)	⬇️
src/lib/misc/cryptobox/cryptobox.cpp	95.23% <0%> (+1.58%)	⬆️
src/lib/pk_pad/emsa_raw/emsa_raw.cpp	74.28% <0%> (+2.85%)	⬆️

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[randombit]

Thanks