Support batch reads in btree implementation

[tomijaga]

Allows us to get multiple entries from the btree, while traversing down the tree only one time.
A function like batch_get() could potentially improve our reads from the main btree that stores the documents. Unlike the secondary index scans where we perform range queries that involve a max of two tree traversals for the lower and upper bounds, reading from the main btree requires us to traverse the tree each time for every document because the document ids are random and have no correlation to the data in the document.