New index txhash -> txnum or its variation

[JkLondon]

Rationale

In case of transaction granularity usage we need index that would provide us txnum by only hash (now we only have txhash->blocknum from e2)

Implementation

There are some variants:

• make new index hash -> txnum
• change existing index to something like hash->txnum,blocknum

It would provide us a way to implement #12343 etc