Changed CMessageHeader::pchCommand to use a C++11 array and updated its various uses to reflect this.

Summary: Addresses T247. Changed CMessageHeader::pchCommand to be implimented as a C++11 array rather than a static array and updated its various uses to reflect this.

Test Plan: make check

Reviewers: deadalnix, jasonbcox, schancel, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1743