bookmarks: use command executor for wire protocol commands
And change the name of a variable to reflect that is is a peer.
Differential Revision: https://phab.mercurial-scm.org/D3292
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL