Add a new configuration variable mu4e-mu-allow-temp-file, t by default,
which triggers the same optimization in the mu server.
It improves message rendering speed:
I.e. without:
[mu4e] Found 500 matching messages; 0 hidden; search: 1298.0 ms (2.60 ms/msg); render: 642.1 ms (1.28 ms/msg)
with:
[mu4e] Found 500 matching messages; 0 hidden; search: 1152.7 ms (2.31 ms/msg); render: 270.1 ms (0.54 ms/msg)
As a useful side-effect, this also:
Fixes#1802.
7c85b61944