RUST-1048 add default_database api for client

[WindSoilder]

closes: #475

Regarding to this:

A new method on Client called default_database(&self) -> Option<&str>
Though, I wonder if this would be more ergonomic if it actually returned an Option

Personally, I think make default_database(&self) returned an Option<Database> is easiler to use. And the behavior is more matches for the function name.

If user want to get default database name, we can get it from ClientOptions.default_database, or using something like client.default_database().unwrap().name().

Maybe something is missing, any review suggestion is appreciated :-)

[patrickfreed]

Thanks for the PR! I have a few minor suggestions and one open question for the team, but otherwise it looks good!

[patrickfreed]

LGTM! (pending green CI run).

Thanks for your contribution!

[WindSoilder]

is there any way to look into why CI failed?

[isabelatkinson]

lgtm mod a few typo fixes!

[patrickfreed]

It looks like rustfmt needs to be run, check out the ./evergreen/check-rustfmt.sh script to see the invocation we use.

The other failures look to be some unrelated known flaky tests, so nothing to worry about there.

[abr-egn]

LGTM! (modulo typo / rustfmt fixes)

[WindSoilder]

Ok, I've fix typo and make a rebase, it should be formatted..

[patrickfreed]

Thanks again for your contribution @WindSoilder!