Add stall detection to the NR java agent

[jtduffy]

Description

Many years ago the java agent had a very useful stall detection feature. The agent would notice transactions that started and had not finished after a certain amount of time. It would then report this as an error with a stack trace for the thread on which the transaction was running.

When it detects a "stall", it will report an event with transaction details similar to those we report in Transaction events. The stack trace will be reported in code.stacktrace.

Initially, configuration will include:

• Enable/disable flag for the new SlowTransactionService
• Configuration to control how often (or at all) previously reported slow transactions get reported

@jack-berg will be handling the implementation. Working branch here.