Added access log to server and modified mock-fts to use it

internal/config/README.md

@@ -101,6 +101,7 @@ The `server` section is split into a number of sub-sections:
 | `server.port` | int | `9393` | Port to listen on; `0`: any free port |
 | `server.max_header_bytes` | byte size | `8KB` | Largest request header block; `0B` for 8KB. `-1` (no limit) is rejected, as the HTTP server always has one |
 | `server.max_body_bytes` | byte size | `1MB` | Largest request body; `0B` for 1MB, `-1` for no limit |
+| `server.access_log` | string | | File the access log is appended to, one line per request in the Combined Log Format; `-`: stdout; empty: no access log. `serve --access-log` overrides it |
 
 #### Timeouts