revlog: rename versionflags to header
The variable is named as such because it contains "version" information and
"flags" information. However you mostly needs to know the code to understand the
name which is not great. The fact that this is the very first four bytes in all
revlog seems more relevant, so we rename the variable "header".
Differential Revision: https://phab.mercurial-scm.org/D10596