@jlporter "I think the first defer_cleanup needs to be for the SerfUrlAsyncFetcher that is created, instead of the RateControllingUrlAsyncFetcher as it is now, since the rate controlling fetcher will be cleaned up by the driver factory."

Isn't that how this works on the current master (without this pull)? 
With a `RateControllingUrlAsyncfetcher`, it seems the defer_cleanup on the `SerfUrlAsyncFetcher` is what is segfaulting (and what this pull intends to fix, but now seems like the wrong fix). 
Backtrace:

```
Program received signal SIGSEGV, Segmentation fault.
0x0000000000000031 in ?? ()
(gdb) bt
#0  0x0000000000000031 in ?? ()
#1  0x0000000000484b99 in net_instaweb::RewriteDriverFactory::Deleter<net_instaweb::SerfUrlAsyncFetcher>::Run (this=<optimized out>)
    at ../ngx_pagespeed/psol/include/net/instaweb/rewriter/public/rewrite_driver_factory.h:475
#2  0x00000000006345c6 in net_instaweb::Function::CallRun (this=0x1217090) at pagespeed/kernel/base/function.cc:51
#3  0x00000000004f65d9 in net_instaweb::RewriteDriverFactory::~RewriteDriverFactory (this=0x11d64b0, __in_chrg=<optimized out>)
    at net/instaweb/rewriter/rewrite_driver_factory.cc:147
#4  0x0000000000484a03 in ~SystemRewriteDriverFactory (this=0x11d64b0, __in_chrg=<optimized out>)
    at ../ngx_pagespeed/psol/include/net/instaweb/system/public/system_rewrite_driver_factory.h:31
#5  net_instaweb::NgxRewriteDriverFactory::~NgxRewriteDriverFactory (this=0x11d64b0, __in_chrg=<optimized out>)
    at ../ngx_pagespeed/src/ngx_rewrite_driver_factory.cc:111
#6  0x0000000000484aad in net_instaweb::NgxRewriteDriverFactory::~NgxRewriteDriverFactory (this=0x11d64b0, __in_chrg=<optimized out>)
    at ../ngx_pagespeed/src/ngx_rewrite_driver_factory.cc:117
#7  0x000000000047c287 in ngx_psol::(anonymous namespace)::ps_cleanup_srv_conf (data=0x120ce68)
    at ../ngx_pagespeed/src/ngx_pagespeed.cc:571
#8  0x000000000040e821 in ngx_destroy_pool (pool=0x11c4440) at src/core/ngx_palloc.c:55
#9  0x000000000042a33b in ngx_master_process_exit (cycle=0x11c4490) at src/os/unix/ngx_process_cycle.c:715
#10 0x000000000042c299 in ngx_single_process_cycle (cycle=0x11c4490) at src/os/unix/ngx_process_cycle.c:325
#11 0x000000000040d709 in main (argc=<optimized out>, argv=<optimized out>) at src/core/nginx.c:409
```