maintenance release before library change - `symplely/processor` to `symplely/spawn`

The next version will use symplely/spawn that uses uv_spawn of libuv for child subprocess management/execution.
The fallback will use same aspects of symplely/processor, if libuv not installed.

• update constants
• insure file operation writes/reads are unbuffered
• update/test file_sendfile for usage without libuv, update file operation headers for uri calls
• update required, fixed to symplely/processor v1.4.3 for subprocess manager