object cache: cleanup strbuf usage

Don't use strbuf for hot path because it relys on the malloc(2) to allocate
memory and will add more presure to the glibc's ptmalloc layer.

This is a defensive patch because we observed segfault of malloc or other glibc
library calls (for e.g, opendir(2)) on top of malloc(2).

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>