core: rename data dir environment var

[kr]

This change serves two goals:

• Rename the env var to CHAIN_CORE_DATA.
• Leave overriding the location to platform-specific
  tooling.

For the second item, the Mac OS X app explicitly sets
CHAIN_CORE_DATA to the appropriate location for
Mac OS X (as opposed to plain Darwin).

Future work: also update the Windows installer.

[kr]

Note: we can't safely land this until the mac app and windows installer are updated. Maybe that should happen in this same PR.

[kr]

This is based on a conversation @dominic and I had earlier today.

[tessr]

I think that the installers should set that environment variable in this PR, too. (I also want to change the Postgres data directory for the windows installer, but I haven't gotten around to it yet.)

[dominic]

Existing change LGTM, but it would be nice to get the Mac app change in here as well.

[edit] I'll aim to push that in here today.

[kr]

rebased

[kr]

[dominic]

I like this, but we've also got https://developer.apple.com/reference/foundation/1413045-nshomedirectory

[dominic]

(We're not sandboxed, but it's probably safest if we ever manage that path.)

[dominic]

Mac LGTM. I had to delete the existing /Application Support folder to get it to run: it wasn't happy with the data there from my other Chain Cores:

at=raft.go:505 t=2017-04-21T23:53:33.516637877Z message="dummy write failed with context deadline exceeded"
raft2017/04/21 16:53:33 INFO: 1 is starting a new election at term 8
raft2017/04/21 16:53:33 INFO: 1 became candidate at term 9
raft2017/04/21 16:53:33 INFO: 1 received MsgVoteResp from 1 at term 9
raft2017/04/21 16:53:33 INFO: 1 became leader at term 9
raft2017/04/21 16:53:33 INFO: raft.node: 1 elected leader 1 at term 9
at=raft.go:683 t=2017-04-21T23:53:33.551906401Z index=73 ent="{9 73 EntryNormal [] []}"
at=raft.go:717 t=2017-04-21T23:53:33.551945752Z EntryNormal="{9 73 EntryNormal [] []}"
at=raft.go:683 t=2017-04-21T23:53:33.570979866Z index=74 ent="{9 74 EntryNormal [123 34 87 99 116 120 34 58 34 105 49 118 72 81 109 53 115 99 43 51 114 55 98 109 79 101 76 76 69 82 65 61 61 34 44 34 73 110 115 116 114 117 99 116 105 111 110 34 58 34 69 103 48 83 67 121 57 107 100 87 49 116 101 86 100 121 97 88 82 108 34 125] []}"
at=raft.go:717 t=2017-04-21T23:53:33.571043271Z EntryNormal="{9 74 EntryNormal [123 34 87 99 116 120 34 58 34 105 49 118 72 81 109 53 115 99 43 51 114 55 98 109 79 101 76 76 69 82 65 61 61 34 44 34 73 110 115 116 114 117 99 116 105 111 110 34 58 34 69 103 48 83 67 121 57 107 100 87 49 116 101 86 100 121 97 88 82 108 34 125] []}"
at=state.go:93 t=2017-04-21T23:53:33.571095800Z message="state instruction: operations:<key:\"/dummyWrite\" > "
2017-04-21 16:53:33.999274-0700 Chain Core[95238:74853139] ServerManager: Chain Core is running.
2017-04-21 16:53:34.573112-0700 Chain Core[95238:74853849] Chain Core stopped unexpectedly. Exiting.