Change mmap area protections to only PROT_READ once read-only

[Thomasdezeeuw]

Benchmark if its worth it to change the protections from PROT_READ | PROT_WRITE to only PROT_READ once all the data to a mmap(2) area have been written, in the Data file. If the page is marked PROT_READ the kernel might be able to optimise sending it into a socket for example. But I don't know if the kernel can/does take advantage of that, so benchmark first.