new API for TxReportQueue (fixes #128)

make an idiomatic version of the read size of the java.util.concurrent.BlockingQueue interface.
ditch `remove` and `element` methods from TxReportQueue

based on discussion https://github.com/pellucidanalytics/datomisca/pull/132#discussion-diff-18282174
Merge pull request #132 from dwhjames/topic/tx-report-queue

new API for TxReportQueue (fixes #128)