key-value store. fixed a bug of abstract method.

origin of bug is commit of 4f4ce383b8df541a60ba7d62e7626d3e34b29888.
bug file was `kv.rb'. the file was renamed to `kvs.rb'.
basic method name was changed. but its call name in abstract method was not changed.
gdbm key-value store. unit test is defined.
hash based key-value store is defined for unit test.

key-value stores in unit test are replaced to hash-kvs from pseudo gdbm-kvs using hash.
changelog:
* lib/rims/kvs.rb: key-value store. fixed a bug of abstract method.
key-value store. added destroy API.

abstract key-value store. defined destroy API.
hash key-value store. implemented destroy API. nothing to do.
gdbm key-value store. implemented destroy API. delete closed gdbm file.
mail store. mailbox DB is destroyed on deletion.
changelog: * lib/rims/kvs.rb, lib/rims/gdbm_kvs.rb, lib/rims/db.rb, lib/rims/mail_store.rb: destroy API is defined at key-value store and mailbox DB file is removed on mailbox deletion.
Merge branch 'kvs_destroy_API'

Conflicts:
	ChangeLog