Optimize restore_savepoint()

[cberner]

This restores and fixes the optimization that was removed in baa86e7

restore_savepoint() now scales with the number of database modifications since the savepoint was captured, rather than the size of the database

savepoint_benchmark is now ~35x faster for an 8GiB database file

Benchmarks:
master:

+---------+----------+------------------------+---------------------+
| DB size | insert() | persistent_savepoint() | restore_savepoint() |
+===================================================================+
| 129MiB  | 7693ns   | 685us                  | 18807us             |
|---------+----------+------------------------+---------------------|
| 257MiB  | 7456ns   | 510us                  | 62092us             |
|---------+----------+------------------------+---------------------|
| 514MiB  | 7126ns   | 379us                  | 152715us            |
|---------+----------+------------------------+---------------------|
| 1028MiB | 7678ns   | 427us                  | 346066us            |
|---------+----------+------------------------+---------------------|
| 2056MiB | 11429ns  | 1402us                 | 1173873us           |
|---------+----------+------------------------+---------------------|
| 4096MiB | 17383ns  | 2169us                 | 2946756us           |
|---------+----------+------------------------+---------------------|
| 8193MiB | 18706ns  | 672us                  | 5288360us           |
+---------+----------+------------------------+---------------------+

Optimized (this PR):

+---------+----------+------------------------+---------------------+
| DB size | insert() | persistent_savepoint() | restore_savepoint() |
+===================================================================+
| 129MiB  | 7153ns   | 656us                  | 2097us              |
|---------+----------+------------------------+---------------------|
| 257MiB  | 7613ns   | 498us                  | 3689us              |
|---------+----------+------------------------+---------------------|
| 514MiB  | 7930ns   | 520us                  | 8685us              |
|---------+----------+------------------------+---------------------|
| 1028MiB | 9116ns   | 679us                  | 23494us             |
|---------+----------+------------------------+---------------------|
| 2056MiB | 12111ns  | 1622us                 | 33498us             |
|---------+----------+------------------------+---------------------|
| 4096MiB | 18542ns  | 2163us                 | 67417us             |
|---------+----------+------------------------+---------------------|
| 8193MiB | 20877ns  | 1310us                 | 145174us            |
+---------+----------+------------------------+---------------------+