Make lsn_reset ("detach databases") optional and off by default.

Add an option -detachdb (and entry in OptionDialog), without which no
lsn_reset is called on addr.dat and blkindex.dat. That means these
files cannot be moved to a new environment, but shutdown can be
significantly faster. The wallet file is always lsn_reset'ed.

-detachdb corresponds to the old behaviour, though it is off by
default now to speed up shutdowns.