Improve the picture in raftable README.

Author: kvap

contrib/raftable/README

@@ -16,23 +16,17 @@ number becomes at least V and returns 'ok' to the frontend.
 
 The backend can also issue commands to itself through C API.
 
-       fork        ┏━━━━━━━━━┓  sql  ┏━━━━━━━━━━┓
-     ┌─────────────┨ backend ┠───────┨ frontend ┃
-     │             ┗┯━━━━━━━┯┛       ┗━━━━━━━━━━┛
-     │              │shm    │
-┏━━━━┷━━━━━┓    ┏━━━┷━━━┓   │
-┃ postgres ┠────┨ state ┃   │
-┗━━━━┯━━━━━┛    ┗━━━┯━━━┛   │ tcp
-     │              │shm    │
-     │ fork    ┏━━━━┷━━━━━┓ │
-     └─────────┨ raftable ┃ │
-               ┗━┯━┯━┯━┯━┯┛ │
-                 │ raft│ │  │
-               ┏━┷━┷━┷━┷━┷┓ │
-              ┏┻━━━━━━━━━┓┠─┘
-             ┏┻━━━━━━━━━┓┣┛
-             ┃ raftable ┣┛
-             ┗━━━━━━━━━━┛
+┓   ┏━━━━━━━━━┓┏━━━━━━━━━┓┏━━━━━━━━━┓   ┏━━━━━━━━━┓┏━━━━━━━━━┓┏━━━━━━━━━┓   ┏
+┃   ┃ backend ┃┃ backend ┃┃ backend ┠─┐ ┃ backend ┃┃ backend ┃┃ backend ┃   ┃
+┛   ┗━━━━┯━━━━┛┗━━━━┯━━━━┛┗━━━━┯━━━━┛ │ ┗━━━━┯━━━━┛┗━━━━┯━━━━┛┗━━━━┯━━━━┛   ┗
+╗   ╔════╧══════════╧══════════╧════╗ T ╔════╧══════════╧══════════╧════╗   ╔
+║   ║    state in shared memory     ║ C ║    state in shared memory     ║   ║
+╝   ╚═══════════════╤═══════════════╝ P ╚═══════════════╤═══════════════╝   ╚
+┓   ┏━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┓ │ ┏━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┓   ┏
+┃   ┃        raftable worker        ┃ └─┨        raftable worker        ┃   ┃
+┛   ┗━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┛   ┗━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┛   ┗
+                    ┆                                   ┆
+╴╴╴╴╴╴╴╴╴╴╴╴╴╴╴╴╴╴╴╴┴╶╶╶╶╶╶╶╶╶╶ Raft over UDP ╴╴╴╴╴╴╴╴╴╴┴╶╶╶╶╶╶╶╶╶╶╶╶╶╶╶╶╶╶╶╶
 
 C API:
 	/* Gets value by key. Returns the value or NULL if not found. */