Panicking when a write txn tries to free a page which was allocated by itself

[ahrtr]

A writing TXN should never free a page which was allocated by itself.

Link to #789

[tjungblu]

testcase that triggers this panic:

https://github.com/etcd-io/bbolt/compare/main...tjungblu:cache_freelist_inconsistency?expand=1#diff-3d69c5fb9cdafd3a4e75384354f4a9e14b5318b0e4ec549000cfebd1ac3c6b1bR183-R202

[ahrtr]

https://github.com/etcd-io/bbolt/compare/main...tjungblu:cache_freelist_inconsistency?expand=1#diff-3d69c5fb9cdafd3a4e75384354f4a9e14b5318b0e4ec549000cfebd1ac3c6b1bR183-R202

line 194, it tries to free a page which was just allocated by itself. You can do this in a unit test, but actually it will never happen in real production use case.

f.free(common.Txid(2), common.NewPage(allocate, common.LeafPageFlag, 0, 1))

[tjungblu]

I'm not saying it's a production issue, I was merely testing all the branches ;-)
If it's not allowed, the panic is the correct way to continue with this.

[tjungblu]

/lgtm

[ahrtr]

Just rebased this PR, PTAL @fuweid @tjungblu

[tjungblu]

still looks good!

/lgtm

[ahrtr]

Just rebased this PR, PTAL @fuweid @tjungblu

[tjungblu]

/lgtm

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