index: Fix coinstatsindex overflow issue

The index originally stored cumulative values in a CAmount type but this allowed for
potential overflow issues which were observed on Signet. Fix this by
storing the values that are in danger of overflowing in a arith_uint256.

Also turns an unnecessary copy into a reference in RevertBlock and
CustomAppend and gets
rid of the explicit total unspendable tracking which can be calculated
by adding the four categories of unspendables together.

doc/files.md

@@ -57,7 +57,7 @@ Subdirectory       | File(s)               | Description
 `indexes/txindex/` | LevelDB database      | Transaction index; *optional*, used if `-txindex=1`
 `indexes/blockfilter/basic/db/` | LevelDB database      | Blockfilter index LevelDB database for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
 `indexes/blockfilter/basic/`    | `fltrNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Blockfilter index filters for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
-`indexes/coinstats/db/` | LevelDB database | Coinstats index; *optional*, used if `-coinstatsindex=1`
+`indexes/coinstatsindex/db/` | LevelDB database | Coinstats index; *optional*, used if `-coinstatsindex=1`
 `wallets/`         |                       | [Contains wallets](#multi-wallet-environment); can be specified by `-walletdir` option; if `wallets/` subdirectory does not exist, wallets reside in the [data directory](#data-directory-location)
 `./`               | `anchors.dat`         | Anchor IP address database, created on shutdown and deleted at startup. Anchors are last known outgoing block-relay-only peers that are tried to re-connect to on startup
 `./`               | `banlist.json`        | Stores the addresses/subnets of banned nodes.