Handle relation DROPs in page server.

Add back code to parse transaction commit and abort records, and in
particular the list of dropped relations in them. Add 'put_unlink'
function to the Timeline trait and implementation. We had the code to
handle dropped relations in the GC code and elsewhere in ObjectRepository
already, but there was nothing to create the RelationSizeEntry::Unlink
tombstone entries until now. Also add a test to check that GC correctly
removes all page versions of a dropped relation.

Implements https://github.com/zenithdb/zenith/issues/232, except for the
"orphaned" rels.

Reviewed-by: Konstantin Knizhnik

pageserver/src/repository.rs

@@ -76,6 +76,9 @@ pub trait Timeline: Send + Sync {
     /// Truncate relation
     fn put_truncation(&self, rel: RelTag, lsn: Lsn, nblocks: u32) -> Result<()>;
 
+    /// Unlink object. This method is used for marking dropped relations.
+    fn put_unlink(&self, tag: RelTag, lsn: Lsn) -> Result<()>;
+
     /// Remember the all WAL before the given LSN has been processed.
     ///
     /// The WAL receiver calls this after the put_* functions, to indicate that