Split work of bgwriter between 2 processes: bgwriter and checkpointer.

bgwriter is now a much less important process, responsible for page
cleaning duties only. checkpointer is now responsible for checkpoints
and so has a key role in shutdown. Later patches will correct doc
references to the now old idea that bgwriter performs checkpoints.
Has beneficial effect on performance at high write rates, but mainly
refactoring to more easily allow changes for power reduction by
simplifying previously tortuous code around required to allow page
cleaning and checkpointing to time slice in the same process.

Patch by me, Review by Dickson Guedes

src/backend/bootstrap/bootstrap.c

@@ -315,6 +315,9 @@ AuxiliaryProcessMain(int argc, char *argv[])
 			case BgWriterProcess:
 				statmsg = "writer process";
 				break;
+			case CheckpointerProcess:
+				statmsg = "checkpointer process";
+				break;
 			case WalWriterProcess:
 				statmsg = "wal writer process";
 				break;
@@ -415,6 +418,11 @@ AuxiliaryProcessMain(int argc, char *argv[])
 			BackgroundWriterMain();
 			proc_exit(1);		/* should never return */
 
+		case CheckpointerProcess:
+			/* don't set signals, checkpointer has its own agenda */
+			CheckpointerMain();
+			proc_exit(1);		/* should never return */
+
 		case WalWriterProcess:
 			/* don't set signals, walwriter has its own agenda */
 			InitXLOGAccess();