QPID-5484 [linearstore] Poor performance relative to legacystore. Partial fix: The use of /dev/random exhausts the kernel's supply of non-deterministic random data, and calls to read from it block until enough is available. It has been determined that /dev/urandom is good enough for this use-case (Journal serial numbers), and does not suffer from blocking.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1558592 13f79535-47bb-0310-9956-ffa450edef68