Add mempool tracepoints

Tracepoints for added, removed, replaced, and rejected transactions.

The removal reason is passed as string instead of a numeric value, since
the benefits of not having to maintain a redundant enum-string mapping
seem to outweigh the small cost of string generation.  The reject reason
is passed as string as well, although here the string does not have to
be generated but is readily available.

contrib/tracing/README.md

@@ -286,3 +286,49 @@ Spent   a05880b8c77971ed0b9f73062c7c4cdb0ff3856ab14cbf8bc481ed571cd34b83:1
 Added   eb689865f7d957938978d6207918748f74e6aa074f47874724327089445b0960:0            5589696005 2094513 No
 Added   eb689865f7d957938978d6207918748f74e6aa074f47874724327089445b0960:1               1565556 2094513 No
 ```
+
+### mempool_monitor.py
+
+A BCC Python script producing mempool statistics and an event log. Based on the
+`mempool:added`, `mempool:removed`, `mempool:replaced`, and `mempool:rejected`
+tracepoints.
+
+Statistics include incidence and rate for each event type since the script was
+started (`total`) as well as during the last minute (`1 min`) and ten minutes
+(`10 min`). The event log shows mempool events in real time, each entry
+comprising a timestamp along with all event data available via the event's
+tracepoint.
+
+```console
+$ python3 contrib/tracing/log_utxocache_flush.py ./src/bitcoind
+```
+
+```
+ Mempool Monitor
+ Press CTRL-C to stop.
+
+ ┌─Event count───────────────────────┐  ┌─Event rate──────────────────────────┐
+ │ Event       total   1 min  10 min │  │ Event       total    1 min   10 min │
+ │ added      4130tx   218tx  2211tx │  │ added     3.9tx/s  3.6tx/s  3.7tx/s │
+ │ removed     201tx     5tx    90tx │  │ removed   0.2tx/s  0.1tx/s  0.1tx/s │
+ │ replaced    201tx     5tx    90tx │  │ replaced  0.2tx/s  0.1tx/s  0.1tx/s │
+ │ rejected     31tx     5tx    23tx │  │ rejected  0.0tx/s  0.1tx/s  0.0tx/s │
+ └───────────────────────────────────┘  └─────────────────────────────────────┘
+
+ ┌─Event log───────────────────────────────────────────────────────────────────────────────────────────────────────────┐
+ │ 17:09:56Z event=added, txid=0a2f2ef84d77a13162f26bb7a544efbab7f09d0b8a965b6f00e85bbd64cd8152, vsize=183, fee=4968   │
+ │ 17:09:56Z event=removed, txid=0356df4450c76d2474eab13fab15bfab843e31617edf9ce88d5d5bea5b19bf3e, reason=replaced, vsi│
+ │ ze=110, fee=16655                                                                                                   │
+ │ 17:09:56Z event=removed, txid=653db4f9af156626a7359c18f9cb095009583497ef606d7a9d2f093c63d392da, reason=replaced, vsi│
+ │ ze=110, fee=4037                                                                                                    │
+ │ 17:09:56Z event=rejected, txid=1cd12698577446904e8654372421e1ab5b82bdb8faee3f11f34ed758c17ae85a, reason=bad-txns-inp│
+ │ uts-missingorspent, peer_id=2568, peer_addr=158.69.116.169:8333                                                     │
+ │ 17:09:56Z event=replaced, replacement_txid=db3c91d7dfff65ea5b85ca58bc3e9a26556eb6506ccb04a573f920d297060218, replace│
+ │ ment_vsize=110, replacement_fee=23343, replaced_txid=0356df4450c76d2474eab13fab15bfab843e31617edf9ce88d5d5bea5b19bf3│
+ │ e, replaced_vsize=110, replaced_fee=16655                                                                           │
+ │ 17:09:56Z event=replaced, replacement_txid=9b304b9357df5e2cfcbbbead7ffa67c0efc3c599aac9093362b398478d46a587, replace│
+ │ ment_vsize=110, replacement_fee=4268, replaced_txid=653db4f9af156626a7359c18f9cb095009583497ef606d7a9d2f093c63d392da│
+ │ , replaced_vsize=110, replaced_fee=4037                                                                             │
+ │                                                                                                                     │
+ └─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
+```