RFC: patches used by DataKit

[talex5]

For DataKit, we apply these patches to ocaml-9p. I'm not sure we want them merged upstream in this form, but I'm making this PR to keep track of them:

• When logging exceptions, include the 9p request that triggered it for debugging. Maybe the signature of exn_converter should be changed instead?

• While a connection is open, send a ping message (walk to /) every 30s. This avoids problems in some networks where TCP connections may silently stop working after a while unless there is traffic to keep the connection alive. It also reports these ping times as prometheus metrics. This should probably be optional.

[talex5]

Note: Travis says:

Test Successful in 0.003s. 27 tests run.
pkg.ml: [ERROR] Some tests failed.

This is because the test executable reports a failure if anything is logged at error level, and the new logging causes this to be logged from failed remove should clunk the fid in lofs_test.ml:

ERR [server] Uncaught exception handling ((tag (1))
                                          (payload (Remove ((fid 1))))): 
             Exception: Unix.Unix_error(Unix.ENOTEMPTY, "rmdir", "/Users/tal/src/ocaml-9p/tmp/foo")Fatal error: exception (Invalid_argument "index out of bounds")

Possibly this is a genuine bug and we should be catching and handling the error rather than throwing an exception.

[djs55]

These patches have been merged as part of #122 and #123 so I'll close this one. Don't forget to use the new optional argument of connect.