Indexer could use smaller hash indexes instead of btree indexes

## Problem

All indexer indices are btree but not all have to be.

## Solution

Replace btree indexes with hash indexes in places where only exact searches are used and the resulting hash index is significantly smaller.

One such example could be :
```
ledgerdb=# create index concurrently txn_by_tixid_hash on txn using hash (txid);
CREATE INDEX

ledgerdb=# \di+
List of relations
Schema | Name | Type | Owner | Table | Size | Description
--------+-------------------------------+-------+-------+-------------------+---------+-------------
public | txn_by_tixid | index | algo | txn | 43 GB |
public | txn_by_tixid_hash | index | algo | txn | 16 GB |
```
## Urgency

Performance optimization.