MINOR: pool: move pool declarations to read_mostly

All pool heads are accessed via a pointer and should not be shared with
highly written variables. Move them to the read_mostly section.
diff --git a/src/hpack-tbl.c b/src/hpack-tbl.c
index da81d29..8df1031 100644
--- a/src/hpack-tbl.c
+++ b/src/hpack-tbl.c
@@ -99,7 +99,7 @@
 	[61] = { .n = IST("www-authenticate"),             .v = IST("")              },
 };
 
-struct pool_head *pool_head_hpack_tbl = NULL;
+struct pool_head *pool_head_hpack_tbl __read_mostly = NULL;
 
 #ifdef DEBUG_HPACK
 /* dump the whole dynamic header table */