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PARQUET-799: Fix bug in MemoryMapSource::CloseFile

The key change here is to ensure that munmap is called at most once for
a given memory-mapped file.

Previously, MemoryMapSource::CloseFile was calling munmap on every
invocation (provided that a file had ever been successfully mapped into
memory for that instance). This is problematic in a multi-threaded
environment, even if each MemoryMapSource instance is being used in only
one thread, as illustrated by the following hypothetical sequence of
operations:

thread 1 @ time 1: munmap(0xf000, 4096)
thread 2 @ time 2: void *addr = mmap(NULL, 4096, ...) // addr <- 0xf000
thread 1 @ time 3: munmap(0xf000, 4096)

After time 3, the mapping for the memory segment beginning at 0xf000 has
been invalidated, so the next attempt by thread 2 to access memory
within that segment will likely cause a segfault (unless yet another
thread has mmap'd that segment in the meantime, in which case the
results could be even more interesting, but certainly no better).

Also, I'm adding/modifying a couple comments in header files to mark
"sample" implementations accordingly. This is intended to give API
consumers a heads up as to the intent and level of maturity of those
sections of the codebase.

Author: William Forson <william@gluent.com>

Closes #203 from wdforson/master and squashes the following commits:

782713a [William Forson] Adjust 'sample' code comments
7337551 [William Forson] PARQUET-799: Fix bug in MemoryMapSource::CloseFile

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