Drop sophia content store back end

[garlick]

We've had two back ends for the content store since its inception, but only the sqlite one ever really gets used. The sophia back end requires the sophia database which we embedded. As a back end for the content store, sophia was never really properly tuned to use memory appropriately, and due to the lack of good documentation at the time, probably doesn't handle all error situations appropriately.

Now the embedded libsophia is outdated (issue #548), and is triggering some thread sanitizer warnings (issue #722).

This PR drops libsophia, the content-sophia module, and its sharness test.

[coveralls]

Coverage increased (+0.2%) to 75.655% when pulling 4e343dd on garlick:drop_sophia into 0a7613e on flux-framework:master.

[grondo]

Looks good! Is sophia still mentioned in the flux-content(1) man page?

git grep sophia on this branch might also show some other lingering references. (e.g. you could also remove the -i src/common/libsophia argument in cppcheck.sh if you have not already)

[garlick]

Oh yeah, git grep - I always forget about that one. Thanks, just pushed a couple more changes as a result.

[grondo]

Thanks! I'll merge this once travis is green.

[coveralls]

Coverage increased (+0.2%) to 75.639% when pulling 6b09973 on garlick:drop_sophia into 0a7613e on flux-framework:master.