Merge #14718: Remove unreferenced boost headers

Summary:
c54e5a41c4 Remove unreferenced boost headers (Murray Nesbitt)

Pull request description:

Building with clang (e.g. on FreeBSD) is very noisy due to `-Wthread-safety-analysis` warnings regarding boost. This change removes a number of unnecessary boost includes, and silences the rest of the warnings when building with clang. This allows more potentially interesting warnings to surface from the noise.

Tested on FreeBSD 11.2

Tree-SHA512: 5e6a0623188b9be59aeae52866799aefb4c3c9ab5e569b07ee8d43fc92e0b5f1f76b96bb54c35c7043148df84641b4a96927fb71f6eb00460c20cd19cf250900

Backport of Core PR14718 and PR14768 (reverts fs.h changes)

Changes to util/system.cpp were ignored because we diverged here:
https://reviews.bitcoinabc.org/rSTAGING47162673c79c757a9c038c4ddc41fb3022223bde

Test Plan:

ninja
ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5523