Fix 'already borrowed' panic in fetcher

[polachok]

 thread 'main' panicked at 'already mutably borrowed: BorrowError', libcore/result.rs:945:5
 stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at libstd/sys_common/backtrace.rs:59
             at libstd/panicking.rs:207
   3: std::panicking::default_hook
             at libstd/panicking.rs:223
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:402
   5: std::panicking::begin_panic_fmt
             at libstd/panicking.rs:349
   6: rust_begin_unwind
             at libstd/panicking.rs:325
   7: core::panicking::panic_fmt
             at libcore/panicking.rs:72
   8: core::result::unwrap_failed
             at /checkout/src/libcore/macros.rs:26
   9: <futures::future::chain::Chain<A, B, C>>::poll
             at /checkout/src/libcore/result.rs:809
             at /checkout/src/libcore/cell.rs:692
             at /src/tokio-kafka/src/consumer/fetcher.rs:183

[flier]

Thanks, we need more test cases to cover those workflow