Make numRecordsPerBlock configurable

[glennq]

@isnotinvain
Attempt to make numRecordsPerBlock configurable.

When testing locally there's a strange failure for pig even when building from master (at line 166 of TestThriftToPig.java):

Failed tests:   test(com.twitter.elephantbird.pig.util.TestThriftToPig): expected:<...,{(1),(2),(3)})}-{({[}),({(and a one),(and a two)}),({(then a one, two),(three!),(FOUR!!)})}-{zero={}, three={}, two={(1,Wait.),(2,What?)]}}> but was:<...,{(1),(2),(3)})}-{({[(and a one),(and a two)}),({}),({(then a one, two),(three!),(FOUR!!)})}-{zero={}, two={(1,Wait.),(2,What?)}, three={]}}>

Passed all tests before this failure.

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