Increase ramBufferSizeMB from 32 to 100 and make maximum heap size configurable.

This makes Solr use more memory and increases performance. Details at
available at https://issues.apache.org/jira/browse/SOLR-4074. I considered
setting it to 320, which is what I did somewhere else, but apparently
there are deminishing returns when using values over 100.

Making the maximum heap configurable is required so we have spurious failures
on machines without enough ram.  We're defaulting to using 1G in labs and ttm
and 4G everywhere else.

Bug: 48164
Change-Id: I61d03705a80f67762091ea3bde8fc2bbcc6ce520
Merge "Increase ramBufferSizeMB from 32 to 100 and make maximum heap size configurable." into production