cache: Make Rows() return a map of UUID to Models

[dave-tucker]

This makes Rows() return a map of UUID to Model.
These models are copies of those in the cache at the time Rows() was
called.

[coveralls]

Pull Request Test Coverage Report for Build 1343352222

• 8 of 12 (66.67%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 72.858%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cache/cache.go	4	8	50.0%

Files with Coverage Reduction	New Missed Lines	%
client/client.go	1	67.64%

Totals
Change from base Build 1343346118:	0.02%
Covered Lines:	4056
Relevant Lines:	5567

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💛 - Coveralls

[dave-tucker]

/cc @trozet this fixes one of the panics you found

[trozet]

@dave-tucker should the cache read have an RLock instead?

[trozet]

oh I see there is an RLock inside Rows and Row, but not in the API call

[dave-tucker]

@trozet addressed this. rather than checking that elem != nil, Rows() now gives a copy of the cache, so it will never contain nil elements and therefore won't panic.

[trozet]

inline comment, otherwise lgtm

[dave-tucker]

@trozet removed the unncecessary calls to model.Clone