[Regression] Max purge sequence number is not updated during compaction if drop deletes flag is set to 1.

with recent changes when the drop deletes flag is set to 1, we simply drop the items without updating the max purged sequence number.
if (ctx->drop_deletes)

{ // all deleted items must be dropped ... return COUCHSTORE_COMPACT_DROP_ITEM; // ...unconditionally }

this is regression from a few builds ago. we should update max purged sequence number when drop deletes flag is both 0 and 1.