ui: Improve ui-dev-server latency

- Make dev server multithreaded
- Don't bother scanning infra directory for changes.
- Avoid computing the abspath all the time

These last two reduce scan time from ~150ms to ~15ms on my machine
ninja -C foo ui on a clean build is 60ms so this is better than just
always running ninja.

Change-Id: I5ccff40e816aa66046dc3b5392e3e3a9ff35dbcd
diff --git a/ui/run-dev-server b/ui/run-dev-server
index 11a602e..9faf567 100755
--- a/ui/run-dev-server
+++ b/ui/run-dev-server
@@ -39,6 +39,7 @@
   -i $ROOT_DIR/src/traced \
   -i $ROOT_DIR/buildtools \
   -i $ROOT_DIR/out \
+  -i $ROOT_DIR/infra \
   -i $ROOT_DIR/ui/node_modules \
   -s $UI_OUT_DIR \
   "$ROOT_DIR/tools/ninja -C $OUT_DIR ui"