PR-1129: Rate limiting intro.

Simple I/O rate limiting intro. Bridging resource utilization across a
group of independent tapdisks.

Comprising:

 - block-valve: Top-level filter issuing bandwith requests, deferring
   I/O to acknowldgement. Usage: valve:/path/to/sock/un

 - td-rated: Stand alone bridge process, listing to bandwidth
   requests, typically from valve:/ instances. Includes a plugin
   interface for various rate limiting algorithms.

Algorithms (yet slightly experimental):

 - "Token Bucket". A classic, with some trivial modifications to
   promote batching.

 - "Meminfo". Watching /proc/meminfo for pagecache congestion.

Signed-off-by: Daniel Stodden <[email protected]>

Author: Daniel Stodden

.hgignore

@@ -21,6 +21,7 @@
 ^drivers/tapdisk-client$
 ^drivers/tapdisk-diff$
 ^drivers/tapdisk-stream$
+^drivers/td-rated$
 ^vhd/vhd-index$
 ^vhd/vhd-update$
 ^vhd/vhd-util$

drivers/Makefile

@@ -3,7 +3,7 @@ include $(BLKTAP_ROOT)/Rules.mk
 
 LIBVHDDIR  = $(BLKTAP_ROOT)/vhd/lib
 
-IBIN       = tapdisk2 td-util tapdisk-stream tapdisk-diff
+IBIN       = tapdisk2 td-util td-rated tapdisk-stream tapdisk-diff
 LOCK_UTIL  = lock-util
 INST_DIR   = /usr/sbin
 
@@ -54,6 +54,7 @@ BLK-OBJS  := block-aio.o
 BLK-OBJS  += block-ram.o
 BLK-OBJS  += block-cache.o
 BLK-OBJS  += block-vhd.o
+BLK-OBJS  += block-valve.o
 BLK-OBJS  += block-vindex.o
 BLK-OBJS  += block-lcache.o