ARROW-1405: [Python] Expose LoggingMemoryPool in Python API

[wesm]

I removed some output of pointer contents in the LoggingMemoryPool implementation which was showing up as garbage in the console. Example:

n [1]: import pyarrow as pa

In [2]: import pyarrow.parquet as pq

In [3]: pa.log_memory_allocations(True)

In [4]: t = pq.read_table('/home/wesm/Downloads/part-00000-6570e34b-b42c-4a39-8adf-21d3a97fb87d.snappy.parquet')
Allocate: size = 320
Allocate: size = 64
Allocate: size = 192
Free: size = 192
Allocate: size = 320
Allocate: size = 64
Allocate: size = 192
Allocate: size = 192
Allocate: size = 1152
Allocate: size = 64
Allocate: size = 64
Allocate: size = 64
Reallocate: old_size = 64 - new_size = 128
Reallocate: old_size = 128 - new_size = 192
Reallocate: old_size = 192 - new_size = 320
Reallocate: old_size = 320 - new_size = 576
Reallocate: old_size = 64 - new_size = 128
Reallocate: old_size = 576 - new_size = 1088
Reallocate: old_size = 128 - new_size = 256

[wesm]

+1