[Python] Review GIL release and acquisition in to_pandas conversion

[asfimport]

There are some places where we should be acquiring the GIL, and we may not be consistently releasing the GIL.

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• Arrow 0.3 release (blocks)
• Align Date implementation between Java and C++ (relates to)

_{Note: This issue was originally created as ARROW-431. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
This was discussed in the PR for ARROW-420

[asfimport]

Wes McKinney / @wesm:
This is included in #575

[asfimport]

Wes McKinney / @wesm:
Resolved by 909f826