Add optional JSON status output

[pianohacker]

trafficstars

Closes #7.

[pianohacker]

@AltSysrq , this does most of what we described. There's three main issues with it:

1. There's file mode changes in the output I don't quite understand.
2. Many error messages that cause the sync to completely fail end up as panics. Is the solution to this to convert all of these to calls to a method on the LoggerImpl objects? That might be a good followup PR.
3. Most of the specific subtypes of conflict log messages don't appear in the log output. This matters more for completeness than my desired use case.

[pianohacker]

There's file mode changes in the output I don't quite understand.

Can you elaborate?

e.g., at line of 335 of test/previous-outputs/1.0.1.json, there's a mode change from 384 to 33152.

panics … LoggerImpl

I didn't think there were any non-catastrophic situations where the exitsing code would panic like that, and I didn't see any situations where your new code would cause that?

Hm, taking a second look, not panics, but things that are returned as errors rather than being logged. This applies to most of the errors from run_sync().

[AltSysrq]

e.g., at line of 335 of test/previous-outputs/1.0.1.json, there's a mode change from 384 to 33152.

That was actually a bug in ensync put where it failed to mask off the non-permission mode bits. I fixed it on master.

[AltSysrq]

things that are returned as errors rather than being logged. This applies to most of the errors from run_sync()

Could that just be handled specially here?