revlog: compress sidedata in `_writeentry`
When appropriate we store the sidedata in a compressed form.
Differential Revision: https://phab.mercurial-scm.org/D10655
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL