fees: move fee_estimates.dat into fees directory

Move block policy fee estimates from fee_estimates.dat to
fees/block_policy_estimates.dat.

On startup, migrate the legacy file to the new path when only the legacy
file exists. If both files exist, keep the new file and remove the
legacy file.

Rename the block policy estimator args source files to the generic
estimator_args.{cpp,h} names and rename FeeestPath to
BlockPolicyFeeEstPath while the path helper is moved into the shared fee
estimator argument code.

doc/files.md

@@ -54,6 +54,7 @@ Subdirectory       | File(s)               | Description
 `blocks/`          | `revNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Block undo data (custom format)
 `blocks/`          | `xor.dat`             | Rolling XOR pattern for block and undo data files
 `chainstate/`      | LevelDB database      | Blockchain state (a compact representation of all currently unspent transaction outputs (UTXOs) and metadata about the transactions they are from)
+`fees/`            | `block_policy_estimates.dat`   | Stores statistics used to estimate minimum transaction fees required for confirmation
 `indexes/txindex/` | LevelDB database      | Transaction index; *optional*, used if `-txindex=1`
 `indexes/txospenderindex/` | LevelDB database      | Transaction spender index; *optional*, used if `-txospenderindex=1`
 `indexes/blockfilter/basic/db/` | LevelDB database      | Blockfilter index LevelDB database for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
@@ -65,7 +66,6 @@ Subdirectory       | File(s)               | Description
 `./`               | `bitcoin.conf`        | User-defined [configuration settings](bitcoin-conf.md) for `bitcoind` or `bitcoin-qt`. File is not written to by the software and must be created manually. Path can be specified by `-conf` option
 `./`               | `bitcoind.pid`        | Stores the process ID (PID) of `bitcoind` or `bitcoin-qt` while running; created at start and deleted on shutdown; can be specified by `-pid` option
 `./`               | `debug.log`           | Contains debug information and general logging generated by `bitcoind` or `bitcoin-qt`; can be specified by `-debuglogfile` option
-`./`               | `fee_estimates.dat`   | Stores statistics used to estimate minimum transaction fees required for confirmation
 `./`               | `guisettings.ini.bak` | Backup of former [GUI settings](#gui-settings) after `-resetguisettings` option is used
 `./`               | `mempool.dat`         | Dump of the mempool's transactions
 `./`               | `onion_v3_private_key` | Cached Tor onion service private key for `-listenonion` option