use internal functions instead of czmq zdigest and zfile classes

[garlick]

Splitting off of #3617:

Problem: zdigest and zfile czmq classes are used in a couple of places in flux-core, but we have redundant functionaltiy "in house". Now that we have pulled in the czmq containers, we may want to stop using other classes so that we can disassociate ourselves from czmq at some point.

These classes are used in t/kvs/hashtest.c, t/module/parent.c, src/cmd/flux-module.c, src/broker/module.c.

zdigest computes the sha-1 digest for a buffer. We have src/common/libutil/sha1.[ch].

zfile is used in our code base to compute the sha-1 digest for a file. We have src/common/libutil/read_all.[ch].

[chu11]

not sure you tagged the right issue, #3312 seems like the wrong one? did you mean #3617 (also in the other issues you opened)

[chu11]

offline discussion w/ @garlick, perhaps t/kvs/hashtest can just be removed. due to lack of configure support for hashtest's various options, most of it has not been used/tested in quite a long time.