Merge #9517: [refactor] Switched httpserver.cpp to use RAII wrapped libevents.

Summary:
1ae86ec Changed event RAII helper functions to inline to deal with duplicate symbol linker errors. (Karl-Johan Alm)
fd369d2 Switched httpserver.cpp to use RAII wrapped libevents. (Kalle Alm)

Tree-SHA512: 877e431f211024d42a3b0800e860e02833398611433e8393f8d5d4970f47f4bd670b900443678c067fec110c087aaab7dc1981ccbf17f6057676fdbbda89aed9

Backport of Core PR 9517
https://github.com/bitcoin/bitcoin/pull/9517/files

Test Plan:
make check
ninja check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2897