Release cluster configuration during shutdown

> ::findleaks -v
BYTES             LEAKED         VMEM_SEG CALLER
12288                  6 fffffd7ffc35f000 MMAP
12288                  1 fffffd7ffcf6c000 MMAP
12288                  1 fffffd7ffcd6d000 MMAP
12288                  1 fffffd7ffc95c000 MMAP
12288                  1 fffffd7ffc75d000 MMAP
12288                  1 fffffd7ffc55e000 MMAP
------------------------------------------------------------------------
           Total       6 oversized leaks, 73728 bytes

CACHE             LEAKED           BUFCTL CALLER
0000000000485028       1 00000000089d0380 ep.so`MemoryTracker::MemoryTracker+0x6b
00000000004a5028       1 0000000009b92ee0 ep.so`EventuallyPersistentEngine::setClusterConfig+0x5f
000000000048a028     512 00000000098300e0 libstdc++.so.6.0.17`operator new+0x1e
------------------------------------------------------------------------
           Total     514 buffers, 40968 bytes
> 0000000009b92ee0::bufctl -v
            ADDR          BUFADDR        TIMESTAMP           THREAD
                            CACHE          LASTLOG         CONTENTS
         9b92ee0          df8f000    c39fa1a13ad83                4
                           4a5028                0                0
                 libumem.so.1`umem_cache_alloc_debug+0xfd
                 libumem.so.1`umem_cache_alloc+0xb3
                 libumem.so.1`umem_alloc+0x64
                 libumem.so.1`malloc+0x3f
                 ep.so`EventuallyPersistentEngine::setClusterConfig+0x5f
                 ep.so`processUnknownCommand+0x73a
                 ep.so`EvpUnknownCommand+0x38
                 bucket_engine.so`bucket_unknown_command+0x23d
                 default_unknown_command+0xb8
                 process_bin_unknown_packet+0xa9
                 process_bin_packet+0xc6
                 complete_nread+0x182
                 conn_nread+0x48
                 event_handler+0x1ce
                 libevent_core-2.0.so.5.1.9`event_base_loop+0x436

Change-Id: I093d7d7112701e7c23bb9522246489d532668aa6
Reviewed-on: http://review.couchbase.org/38659
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>