* core/rawdb, cmd, ethdb, eth: implement freezer tail deletion

* core/rawdb: address comments from martin and sina

* core/rawdb: fixes cornercase in tail deletion

* core/rawdb: separate metadata into a standalone file

* core/rawdb: remove unused code

* core/rawdb: add random test

* core/rawdb: polish code

* core/rawdb: fsync meta file before manipulating the index

* core/rawdb: fix typo

* core/rawdb: address comments