database: convert Tx.done to atomic type
database: convert Tx.done to atomic type

Change-Id: I9ec725009376f5865adedca6c159b14140dde097
Reviewed-on: https://go-review.googlesource.com/c/go/+/426086
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Reviewed-by: Daniel Theophanes <14db39248b80c6688f7e9808ba5055f2e56426f3@gmail.com>
Run-TryBot: Daniel Theophanes <14db39248b80c6688f7e9808ba5055f2e56426f3@gmail.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go