Fix model commit to database.
Fix model commit to database.

The previous fix (bb7087f) to prevent unnecessary commits to the
database broke storing models when setting IsPersisted (and not changing
anything else - exactly what sync does), so those models would just
hover around in memory until collected by GC.

ZhangLeiCharles/mobile,peeedge/mobile,masterrr/mobile,ZhangLeiCharles/mobile,eatskolnikov/mobile,masterrr/mobile,eatskolnikov/mobile,peeedge/mobile,eatskolnikov/mobile