Attempt to read full local object into the buffer in GET

[cthulhu-rider]

Makes sense, add an issue for that.

Originally posted by @roman-khimov in #3882 (comment)

see TODO

[roman-khimov]

Just as a side note: Head is OK with much smaller buffers, allocating 256K for it is an overkill, but FSTree internals can be adjusted to read whatever the amount provided buffer has in which case we'd get something small for HEAD and 256K for GET. Then the first response chunk can be incomplete (256K-header), to me it's easier than trying to read the remaining kilobytes.