Improved _active_tasks API

Tasks are now free to set any properties they wish (as an
Erlang proplist). Different tasks can have different properties
and the status string doesn't exist anymore - instead client
applications can build it using more granular properties from
_active_tasks. Some of these properties are:

1) "progress" (an integer percentage, for all tasks)
2) "database" (for compactions and indexer tasks)
3) "design_document" (for indexer and view compaction tasks)
4) "source" and "target" (for replications)
5) "docs_read", "docs_written", "doc_write_failures",
   "missing_revs_found", "missing_revs_checked", "source_seq",
   "checkpointed_source_seq" and "continuous" for replications

BugzID: 14269

Conflicts:
	apps/couch/src/couch_db_updater.erl
	apps/couch/src/couch_rep.erl
	apps/couch/src/couch_task_status.erl
	apps/couch/src/couch_view_compactor.erl
	apps/couch/src/couch_view_updater.erl

Conflicts:
	apps/couch/include/couch_db.hrl
	apps/couch/src/couch_db.erl
	apps/couch/src/couch_os_process.erl
	apps/couch/src/couch_query_servers.erl
	apps/couch/src/couch_rep.erl
	apps/couch/src/couch_replication_manager.erl
	apps/couch/src/couch_view_compactor.erl
	apps/couch/src/couch_view_group.erl
	apps/couch/src/couch_view_updater.erl
	configure.ac
	couchjs/c_src/http.c
	couchjs/c_src/main.c
	couchjs/c_src/utf8.c
	etc/windows/couchdb.iss.tpl
	src/couchdb/priv/Makefile.am
	src/couchdb/priv/couch_js/main.c
	test/etap/160-vhosts.t
	test/etap/200-view-group-no-db-leaks.t
	test/etap/Makefile.am

BugzID: 12645