MB-8540 Use new reduce context for compaction retry phase

For view compaction retry phase, setup and tierdown a new
and clean reduce context for each iteration.
Not doing this could lead to crashes during this phase when
a group has at least one view with a JavaScript reduce
function:

=========================CRASH REPORT=========================
  crasher:
    initial call: compaction_daemon:-spawn_view_compactor/5-fun-1-/0
    pid: <0.20220.646>
    registered_name: []
    exception exit: {badarg,
                        [{mapreduce,reduce,
                             [undefined,
                              [{<<"[\"4iB0Tngt7--\",\"u\",\"1\",\"5\",\"s\",\"8\",\"h\",\"y\",\"m\",\"p\",\"z\",\"4\"]">>,
                                <<"\"latitude\"">>},
                               {<<"[\"4iB0Tngt7--\",\"u\",\"1\",\"5\",\"s\",\"8\",\"h\",\"y\",\"m\",\"r\",\"b\",\"6\"]">>,
                                <<"\"latitude\"">>},
                               {<<"[\"4iB0Tngt7--\",\"u\",\"1\",\"5\",\"s\",\"8\",\"h\",\"y\",\"m\",\"r\",\"b\",\"6\"]">>,
                                <<"\"latitude\"">>},
                               {<<"[\"4iB0Tngt7--\",\"u\",\"1\",\"5\",\"s\",\"8\",\"h\",\"y\",\"m\",\"w\",\"4\",\"8\"]">>,
                                <<"\"latitude\"">>}]]},
                         {couch_set_view_mapreduce,reduce,2},
                         {couch_set_view_group,'-init_group/3-fun-3-',7},
                         {couch_btree,'-write_node/3-lc$^0/1-0-',5},
                         {couch_btree,write_node,3},
                         {couch_btree,modify_node,8},
                         {couch_btree,modify_kpnode,10},
                         {couch_btree,modify_node,8}]}
      in function  compaction_daemon:do_chain_compactors/2
    ancestors: [<0.20205.646>,<0.20203.646>,compaction_daemon,<0.2180.433>,
                  ns_server_sup,ns_server_cluster_sup,<0.60.0>]
    messages: []
    links: [<0.20205.646>]
    dictionary: []
    trap_exit: true
    status: running
    heap_size: 6765
    stack_size: 24
    reductions: 10886
  neighbours:

Change-Id: I0613fa66ed4fbc8577c98b3d763ec6c738a907ce
Reviewed-on: http://review.couchbase.org/27217
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>