Keep ScopedEndpointBufferAlloc in variables

This way they don't immediately destruct. Also
up the logging for an allocation failure and use
posix_madvise to possibly improve memory performance.

Test: Transfer files and verify that allocs are scoped properly
Bug: 30976142
Change-Id: If8d2c03cefcc98a83bbb57618c35337529437f73
1 file changed