Use Number instead of NumberLong when indexing in checkWritesOfCommittedTxns

Currently, in multi_statment_transaction_atomicity_isolation.js test, when we go to run checkWritesOfCommittedTxns, this comparison is done incorrectly due to the use of NumberLong to index within the updatedDocsServerHistory array instead of Numbers. The Javascript interpreter instead added the entries for the updatedDocsServerHistory as properties to the array object instead of objects in the array.