Optimize StaticProxyFactory GetProxy and GetFieldInterceptionProxy methods

[maca88]

The optimization reuses the same NHibernateProxyFactoryInfo and ProxyCacheEntry instances in order to reduce memory usage.
Here are some benchmark results from RawDataAccessBencher, before:

Async eager Load fetches
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# of elements fetched: 6768 (4768 + 1000 + 1000).    Fetch took: 93.425.240,00ms. Allocated bytes: 93425240.

Change tracking fetches, set fetches (20 runs), no caching
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NHibernate v5.2.0.0 (v5.2.2.0) : 2.114,26ms (77,83ms)   Enum: 1,79ms (0,23ms)

Memory usage, per iteration
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NHibernate v5.2.0.0 (v5.2.2.0) : 215.322 KB (220.490.360 bytes)

after:

Async eager Load fetches
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# of elements fetched: 6768 (4768 + 1000 + 1000).    Fetch took: 90.673.920,00ms. Allocated bytes: 90673920.

Change tracking fetches, set fetches (20 runs), no caching
------------------------------------------------------------------------------
NHibernate v5.2.0.0 (v5.2.2.0) : 2.050,20ms (58,98ms)   Enum: 1,71ms (0,08ms)

Memory usage, per iteration
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NHibernate v5.2.0.0 (v5.2.2.0) : 195.868 KB (200.569.192 bytes)