Documentation: tuning and modeling guide

[rnewman]

As a relatively flexible piece of infrastructure, rkv/LMDB will benefit from a tuning/usage guide. This will cover the following (and more):

• Versioning parts of your data (see also Thorough support for versioning #7).
• Configuring store sizes and sharing between components.
• Managing key options (dupes, key max sizes, key types, integer keys).
• Avoiding copies where necessary. (Rather than decoding the value, return &[u8].)
• Batching writes, transactions, etc.
• Iterating over keys in order.
• Controlling warming and liveness via background threads.
• WRITEMAP, fsyncs, and other durability/performance/safety tradeoffs.
• Synchronous, asynchronous, and background access.
• Correct use from multiple threads and processes.