Store only last 2 postgres timeline histories

[dineshba]

Fixes #629

We have made changes to store last N (MaxPostgresTimelinesHistory) timeline histories. We have set the default value as 1000.

We introduced an interface PGManager to mock GetTimelinesHistory method in unit test.

Question:

• Does it make sense to make MaxPostgresTimelinesHistory configurable through clusterspec?
• What would be the suitable default value for MaxPostgresTimelinesHistory?

[sgotti]

@dineshba Thanks for the PR!

TimeLineHistory is used to check that the current db is not diverged and is used in function isDifferentTimelineBranch in both keeper.go and sentinel.go (they are quite identical)

Looking at these functions logic we could theoretically just keep the last two timeline histories.

So we could avoid having an user defined (and unclear from the user point of view) parameter and just keep the last N (where N could be just the last 2 or be conservative and choose something like 10...)

[dineshba]

Make sense @sgotti. It couldn't be user defined parameter. We can make N as 2. Will make the changes and update the PR.

[dineshba]

@sgotti Updated the PR with maxPostgresTimelinesHistory = 2(made it keeper.go level const declaration). Squashed and did force push to have clean history. Please refer it.

[sgotti]

@dineshba LGTM. Just a small nit.

[sgotti]

@dineshba Thanks! Merging.