Implement db->pre_acquire_read_lock(), db->dbt_pos_infty(), and db->dbt_neg_infty()
functions.

Example usage to lock entire table:
db->pre_acquire_read_lock(db, txn, db->dbt_neg_infty(), NULL, db->dbt_pos_infty(), NULL);
(Above is for db with no duplicates, i.e. primary index).

For db with duplicates:
db->pre_acquire_read_lock(db, txn, db->dbt_neg_infty(), db->dbt_neg_infty(), db->dbt_pos_infty(), db->dbt_pos_infty());

Will return same lock error codes as db calls do (when locks fail).
0 = success
DB_LOCK_NOTGRANTED = failed to obtain the lock.
We can theoretically sleep and try again here.

If you want to grab from DBT* A to infinity..
db->pre_acquire_read_lock(db, txn, A, NULL, db->dbt_pos_infty(), NULL);


git-svn-id: file:///svn/tokudb@4402 c7de825b-a66e-492c-adef-691d508d4ae1