storageproxyd: discard writes when checkpointing, if necessary

If a checkpointing operation is in progress, discard any write operations
that are flagged as STORAGE_MSG_FLAG_PRE_COMMIT_CHECKPOINT. In tandem
with trusty-side changes that set the flag appropriately, this avoids
the awkward case where the checkpoint is rolled back, which potentially
leads to inconsistency between the data and the superblock.

Based on Stephen's CL/1845477 "Add helper to check checkpoint state of
mounts".

Test: m storageproxyd
Bug: 194313068
Change-Id: Ib6a432db1bc1b034f803b743b0d7322e3f31d814