Commits on Jun 19, 2022

1. POC: use shared ArrayBuffer for nextv() …

   This makes `db.iterator()` with buffer encoding as fast as an
   iterator with utf8 encoding. The approach is:
   
   1. In each `nextv()` call, create a `std::vector<char>` to hold the
      raw data of multiple entries
   2. Copy LevelDB slices directly into that vector with `memcpy()`
   3. Create an ArrayBuffer backed by the vector
   4. In JS, split it into Buffers, each backed by the same ArrayBuffer
      but using a different offset.
   
   Apart from this being an incomplete implementation (it makes utf8
   slower because the C++ side is buffer-only meaning JS has to
   transcode buffer to utf8), the approach has a downside: if userland
   code keeps a reference to just one of the Buffers, the entire
   ArrayBuffer is kept alive too. I.e. it costs memory.
   
   For now this PR is just a reference. The ideal solution (for this
   particular bottleneck) sits somewhere in between. For example, I
   might take just the ArrayBuffer concept, to replace use of
   `napi_create_buffer_copy()`.

   

   vweevers committed Jun 19, 2022
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