According to the BerkeleyDB docs, the DbEnv handle may not be accessed
after close() has been called. This change ensures that we create a new
handle after close() is called. This avoids a segfault when the first
connection attempt fails and then a second connection attempt tries to
call open() on the already closed DbEnv handle.

Instead of having the object destroy itself, having the caller
destroy it.

Adds a ReloadDbEnv function to BerkeleyEnvironment in order to close all Db
instances, closes the environment, resets it, and then reopens
the BerkeleyEnvironment.

Also adds a ReloadDbEnv function to BerkeleyDatabase that calls
BerkeleyEnvironment's ReloadDbEnv.

Calls ReloadDbEnv after encrypting the wallet so that the database
environment is flushed, closed, and reopened to prevent unencrypted
keys from being saved on disk.

Since the database environment is flushed, closed, and reopened during
EncryptWallet, there is no need to shut down the software anymore.

Adaptation of btc@c1dde3a949b36ce9c2155777b3fa1372e7ed97d8

There are tests, like the ones that uses the wallet encryption, that need access to a real wallet database and not to a mock.