Avoid copying data during serialization of Record

[ikopylov]

Currently, the whole data copied to the final vector here: https://github.com/qoollo/pearl/blob/master/src/record.rs#L116
This becomes a problem, when data is large (~1Gb).
We can try to avoid this copy and pass data as a separate parameter to write_append here: https://github.com/qoollo/pearl/blob/master/src/blob/core.rs#L240

There is write_all_vectored function (https://doc.rust-lang.org/std/io/trait.Write.html#method.write_all_vectored) to write 2 buffers in a single call, but it is nightly. Still not copying should provide a significant improvement for large data even with two write calls.

If the data is small (<4Kb) we can use the current approach with single buffer. For large data 2 calls is better

[ikopylov]

Related to #189