Don't seek on imports

(other than the possible seek for custom importers)

We were seeking to handle blob markers. This has two major drawbacks:

1. It wasn't possible to use a non-seekable file.  A use case for
   export/import is to copy database data.  An intermediate file, and
   associated I/O, could be avoided using a pipe, but pipes aren't
   seekable.

2. Seeks cause file-buffer data to be discarded, making IO far more
   expensive.

We didn't really need blob markers, because the preceeding blob data
records serve as markers.  (Now we're stuck with them for backward
compatibility.)

src/ZODB/ExportImport.py

@@ -158,9 +158,15 @@ def persistent_load(ooid):
                 oids[ooid] = oid = self._storage.new_oid()
                 return_oid_list.append(oid)
 
-            # Blob support
-            blob_begin = f.read(len(blob_begin_marker))
-            if blob_begin == blob_begin_marker:
+            if (len(data) < 99 and 'blob' in data and
+                isinstance(self._reader.getGhost(data), Blob)
+                ):
+                # Blob support
+
+                # Make sure we have a (redundant, overly) blob marker.
+                if f.read(len(blob_begin_marker)) != blob_begin_marker:
+                    raise ValueError("No data for blob object")
+
                 # Copy the blob data to a temporary file
                 # and remember the name
                 blob_len = u64(f.read(8))
@@ -169,7 +175,6 @@ def persistent_load(ooid):
                 cp(f, blob_file, blob_len)
                 blob_file.close()
             else:
-                f.seek(-len(blob_begin_marker),1)
                 blob_filename = None
 
             pfile = BytesIO(data)