The max size here is designed around Ruby's Dir::Tmpname.create which
creates temporary filenames in the format

    $TIMESTAMP-$PID-$RANDOM

I believe the previous value of this field was based on the assumption
that PIDs are 1-65535, which isn't necessarily the case on 64 bit Linux
systems, which can be up to 2**22.

    $ uname -a
    Linux zergling 5.4.11-arch1-1 #1 SMP PREEMPT Sun, 12 Jan 2020 12:15:27 +0000 x86_64 GNU/Linux
    $ cat /proc/sys/kernel/pid_max
    4194304

I've chosen a new value based on what I believe the largest possible
tempname is:

    255 - "20200117-4194304-#{0x100000000.to_s(36)}.lock".length #=> 226

(cherry picked from commit a98f330f)