Let set_solver not use eauto by default.

In most cases there is a lot of duplicate proof search performed by
both naive_solver and eauto. Especially since naive_solver calls its
tactic (in the case of set_solver this used to be eauto) quite eagerly
this made it very slow.

Note that set_solver is this too slow and should be improved.
2 jobs for master
Status Job ID Name Coverage
  Test
failed #16
coq
buildjob

00:02:37

failed #6
coq
buildjob

00:02:36

 
Name Stage Failure
failed
buildjob Test There is an unknown failure, please try again
make[1]: *** [barrier/barrier.vo] Error 1
make[1]: *** Waiting for unfinished jobs....
Makefile.coq:322: recipe for target 'barrier/barrier.vo' failed
Closed under the global context
make[1]: Leaving directory '/builds/FP/iris-coq'
make: *** [all] Error 2
Makefile:4: recipe for target 'all' failed

ERROR: Build failed with: exit code 1