Fix encrypted heed3 parallel reads

[Kerollmops]

This PR exposes the bug found in LMDB when trying to read an encrypted database from multiple threads at the same time #336. When running the example in Rust debug I get the following error message which tends to say that the Rust std slice is not happy about the pointer and/or arroy length it receives in this function.

thread '<unnamed>' panicked at library/core/src/panicking.rs:218:5:
unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`

I don't know if @hyc (👋), you have any clue, but it seems to me that the encrypted version of LMDB doesn't like parallel operations. Have I done something wrong somewhere when exposing this feature?

[hyc]

I have no idea at the moment. Multithreaded reads works perfectly well with encryption in OpenLDAP slapd.

Have you tried testing with the example crypto.lm in the LMDB source?