[master] Add new version of GetPendingTxns API using RemoteStorageDB

[ansnunez]

Description

This PR re-introduces GetPendingTxns API, but now we fetch pending txns (i.e., modificationStatus < 2) from remote storage DB instead of using the local pending transaction pool.

The query is:
{ "epochUpdated" : { "$gt" : "<curr epoch - PENDING_TXN_QUERY_NUM_EPOCHS>" }, "epochUpdated" : { "$lte" : "<curr epoch>" }, "modificationState" : { "$lt" : 2 } }

Only the first PENDING_TXN_QUERY_MAX_RESULTS results are fetched. Results are sorted by epochUpdated descending. Only ID and status fields are fetched.

In a way this will be a breaking change as the original GetPendingTxns had no results limit, and also included dropped/rejected transactions.

Backward Compatibility

• This is not a breaking change
• This is a breaking change

Review Suggestion

Status

Implementation

• ready for review

Integration Test (Core Team)

• local machine test

• small-scale cloud test