WT-2693 Check open_cursor error paths for consistent handling (#2859)

* WT-2693 Check open_cursor error paths for consistent handling

There's a problem with cursor functions which don't call their typed
close functions, relying instead on a simple free of the base cursor
type because the __wt_cursor_init() function is the last function called
before successful return. The problem is __wt_cursor_init() doesn't
clean up after itself: for example, it allocates memory to hold the URI
in WT_CURSOR.internal_uri, then has a number of subsequent potential
error returns, and WT_CURSOR.internal_uri won't be free'd unless
__wt_cursor_close() is called. In other words, we should always call
the typed closed function (which in turn calls __wt_cursor_close()).

Specific additional concerns:

cursor/cur_backup.c: __wt_curbackup_open() doesn't turn off backups or
clear the hot-backup flag if __wt_cursor_init() fails, which will hang
the process. Remove the cleanup-on-error code from the __backup_start()
function and rely instead on the backup cursor's close function to do
the work. There's a problem if we fail the initial backup cursor test,
in which case we don't want to do cleanup other than discarding the
cursors themselves. Add the WT_CURBACKUP_LOCKER flag to the
WT_CURSOR_BACKUP structure, which isn't set until we're holding the hot
backup cursor flag, and use it to figure out how much cleanup needs to
be done.

cursor/cur_file.c: the change to call the __curfile_close() function has
a problem with __wt_bulk_wrapup(), which assumes WT_CURSOR_BULK.reconcile
has been initialized, change it to check for NULL.

cursor/cur_file.c: the change to call the __curfile_close() function has
a problem with incrementing the data-source's in-use counter. Instead
of incrementing the counter in __wt_curfile_open() before calling
__curfile_create(), increment it in __curfile_create() after the point
where all failure paths close the cursor.

cursor/cur_log.c: the change to call the __curlog_close() function means
we may be required to clean up a cursor that has not yet acquired the
log archive lock. Add the WT_CURLOG_ARCHIVE_LOCK flag to the WT_CURSOR_LOG
structure so the close function knows if the log archive lock needs to be
released.

cursor/cur_log.c: remove the __curlog_reset() call from __curlog_close(),
we're closing the cursor, resetting it doesn't do anything useful.

cursor/cur_metadata.c: the change to call the __curmetadata_close()
function is slightly tricky, be cautious about closing the underlying
WT_CURSOR_METADATA.file_cursor field, it may not be set when we call
__curmetadata_close().

* comment typo

* fix a comment

* Add explicit comment on the life-cycle of the WT_CURBACKUP_LOCKER flag.

* Review comment, set a specific size on the flags field. Generally, the
cursors all use uint8_t for flags, and I don't think it matters much
what we use, use the same thing everywhere.

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