Fix inconsistency in Partial String Index with 'second' resolution
- Closes #14826
- Tests added: test_partial_string_indexes. Comprehensive testing of partial string indexes.

## Whatsnew entry
Fix inconsistency in Partial String Index with 'second' resolution. See #14826. Now if the timestamp and the index both have resolution `second`, timestamp is considered as an exact match try and not a slice. Therefore, for `Series`, scalar will be returned, for `DataFrame` `KeyError` raised.
I am +1 on the idea of this PR. 
@jreback in the discussion on the issue, I interpreted your comments as opposed to this change. Is that still the case? 
The only problem is whether we should just break this behaviour, or are able to first raise a FutureWarning for this to deprecate it. @jorisvandenbossche no was fine with it. It was a fix designed for DataFrame, but broke Series (which was never tested), so for sure a bug. I don't think there is a problem with simply changing this, it would give the wrong shape back previously. ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14856?src=pr) is 84.64% (diff: 100%)
> Merging [#14856](https://codecov.io/gh/pandas-dev/pandas/pull/14856?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #14856   diff @@
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  Files           144        144          
  Lines         51016      51016          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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- Hits          43185      43184     -1   
- Misses         7831       7832     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [8e630b6...2881a53](https://codecov.io/gh/pandas-dev/pandas/compare/8e630b63010ffdb4ed3c9b47309dc237f8141fdc...2881a5338554b77a21a391795790053cc5a9cbcc?src=pr) I addressed code review comments by @jreback. pls add a whatsnew entry (0.20.0) in other api changes. a small sub-section, showing the change. @jreback I added whatsnew subsection and also edited documentation a little bit. thanks!

pls review the docs when built (will prob take a few hours) and issue a follow up if needed. http://pandas-docs.github.io/pandas-docs-travis/timeseries.html will show them @jreback Thanks for the corrections. Articles are not my strong side :) I created a follow-up PR #14966 to correct a couple of typos.

Btw, this is my first PR merged to large open source project. Thanks for your guiding :)