Add block and reference level consistency checks for reftables

In #228 (closed), we added the basic framework for consistency checks in reftables and also added table level checks.

Continue this with block level and reference level consistency checks:

Block (each block under the table)

• valid block type
• each restart offset should point to a full prefix
• index blocks should point to existing refs/reflogs/objects

References

• symrefs point to existing valid refs (to check)
• valid refname (to check)