Merge bitcoindevkit/bdk#785: Fix wallet export rescan height

e9bbb8724f92ada33228bbfc22acdaefbd79c44e Fix wallet export rescan height (LLFourn)

Pull request description:

  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:

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

ACKs for top commit:
  rajarshimaitra:
    tACK e9bbb8724f92ada33228bbfc22acdaefbd79c44e
  notmandatory:
    ACK e9bbb8724f92ada33228bbfc22acdaefbd79c44e

Tree-SHA512: 9b29ef0df39d26806f48b38fa5c3643bad32f58b993ffdcfc7811aca64a025bd8f163967321f874aa2ef3d29c3e7bc6e2f44d348306a37111f4def036d4c095e