Revert "Use a non-blocking socket for store communication."

This reverts commit 38790945 that causes:

2020-11-02T11:05:08 fatal: uncaught exception 'wrong-type-arg' in 'build' fiber!
2020-11-02T11:05:08 exception arguments: ("struct-vtable" "Wrong type argument in position 1 (expecting struct): ~S" (#f) (#f))
In ice-9/boot-9.scm:
  1731:15 12 (with-exception-handler #<procedure 7fb1a93f9930 at ic…> …)
  1736:10 11 (with-exception-handler _ _ #:unwind? _ # _)
    718:2 10 (call-with-prompt ("break") #<procedure 7fb1ab76f440 a…> …)
    718:2  9 (call-with-prompt ("continue") #<procedure 7fb1ab77084…> …)
In ice-9/eval.scm:
    619:8  8 (_ #(#(#<directory (guile-user) 7fb1ac680f00> #<var…> …)))
In srfi/srfi-1.scm:
    634:9  7 (for-each #<procedure 7fb1a9525900 at cuirass/base.scm…> …)
In ice-9/boot-9.scm:
  1731:15  6 (with-exception-handler #<procedure 7fb1a95a94e0 at ic…> …)
  1669:16  5 (raise-exception _ #:continuable? _)
  1764:13  4 (_ #<&compound-exception components: (#<&assertion-fail…>)
In cuirass/utils.scm:
    319:8  3 (_ _ . _)
In ice-9/boot-9.scm:
  1731:15  2 (with-exception-handler #<procedure 7fb1ab2e3720 at ic…> …)
In cuirass/utils.scm:
   320:22  1 (_)
In unknown file:
           0 (make-stack #t)
ERROR: In procedure make-stack:
In procedure struct-vtable: Wrong type argument in position 1 (expecting struct): #f