Fix clang warnings

Summary:
There is one remaining warning in leveldb.
The unused -pthread warning is removed by D2219.
This diff fix all the other warnings.

Test Plan:
Build with clang and check there is no warning (exceptions are
mentionned above)

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2220