Coding guidelines added in TODO file.
Adjusted test_insert.go to cummulatively insert millions of entries.
Share btree instance among go-routines.

- Using ReadAt() and WriteAt() instead of using Read() and Write() APIs after
  Seek().
  Because we allow file descriptors to be shared among go-routines to access the
  same BTree.
- Cosmetic changes in btree.go
- Snapshot consistency check in defer.go is node under Debug.
While inserting, if (key,docid) is already present entry is overwritten.
Bug in detecting circular loops.
Updated test cases

- for unit-test cases, doc-id is changed to int64 from int.
- test-cases can now share BTree instances across go-routines.