[wip]bt index: read value if key matches straight away

[awskii]

another try to improve bt indices.

• keep cursors in pool
• reduce allocations to zero during seek
• read Value immediately if key matches during Get (skip another ef.Get and decoding key twice)

Reduces load on GC and slightly improves performance

tried to use MatchCmp as well but this causes gas mismatch (i assume during reading from compressed file, because local tests are green though).