(COUCHDB-1305) isolate db process crashes

Merge of 1305-fix-isolate-db-crashes

[@davisp: I'll keep doing this nonsense until we allow merge commits!]

Note that there was previously a small error where couch_sys_dbs ETS
table was being searched for a #db record rather than the db name.

That's fixed here because I saw it. Sorry for not making it a
separate commit.

Squashed commit of the following:

commit f9e4e8a6e426d1569fac4cd707bbd393b102147d
Author: Randall Leeds <randall@apache.org>
Date:   Sat Jan 26 06:28:20 2013 -0800

    don't handle case clause when death happens anyway

    Presumably the reason for the process exit is already logged by
    the exiting process (unlike an unexpected message of another sort
    which may not have been logged already).

commit 10a052a3eddbd7e89b553966895ee38a9ce439d4
Author: Randall Leeds <randall@apache.org>
Date:   Sat Jan 26 06:25:23 2013 -0800

    consolidate two similar handle_info clauses

commit 6f3feb09e347dfe0c6812fd71e3f40d15d8d1ced
Author: Randall Leeds <randall@apache.org>
Date:   Sat Jan 26 04:35:35 2013 -0800

    isolate db process crashes in couch_server

    closes COUCHDB-1305