Fix wallet export rescan height

[LLFourn]

It would return the latest transaction height rather than the earliest as the height to rescan from.

Found by @evanlinjin and I while implementing bdk_core stuff into bdk's wallet.

Changelog notice

• Fix wallet export transaction height

All Submissions:

• I've signed all my commits
• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing

[notmandatory]

The Changelog notice section is where you'd put what ever change log message (if any) you want put in the release notes changelog (which we now manually put together and add to the release tag and on Github releases page).

[rajarshimaitra]

tACK e9bbb87

Yup makes sense to export the "sync from" height rather than last seen height..

[notmandatory]

ACK e9bbb87