Change btree.c to compile with all versions of LMDB

[maxwell-k]

After applying this PR LMDB will compile against all the current versions of LMDB. Versions above 0.9.16 fail as noted in #7

• [x] Try to separate out copyright changes
• [ ] Benchmarking version 2
  • [ ] Outline process to go from source code to gh-pages
  • [ ] Process should keep version 1 as an artefact
  • [ ] Change the versions of SQLite shown in the table
• [ ] Ideas for version 2:
  • [ ] Include raw HTML files as produced by TCL script in gh-pages

Potentially has a negative effect on performance, do not merge until the performance impact has been assessed by sharing bench marking data

[maxwell-k]

Dan commented on the other PR, before I closed it by mistake that he's pretty sure there isn't a performance impact.

I intend to measure the performance impact of this PR before merging.

[maxwell-k]

This can be taken forward with a second version of the benchmarking after https://github.com/LumoSQL/LumoSQL/pull/20 .

Split out the copyright changes into a separate PR.

[maxwell-k]

At the moment I'm going to suggest showing the following in benchmarking version 2

A. sqlite.org 3.30.1 as in version 1 B. sqlite.org 3.31.1 which is from 2020 C. 3.7.17 with LMDB 0.9.16 before applying this patch as in version 1 D. 3.7.17 with LMDB 0.9.16 after applying this patch