Improve PXF CLI reliability

[bradfordb-vmware]

1. Check the status of the PXF service by checking the status of the PXF JVM process and by checking the HTTP health endpoint; if the PXF webapp fails to load for some reason, the JVM exits.

2. Disable actuator shutdown endpoint

   • The recommended way for shutting down a Spring Boot application is to send the JVM SIGTERM. This commit disables the shutdown endpoint since we can always assume that PXF is being shutdown locally and not remotely.

3. Add logging function to PXF CLI

   • PXF CLI writes log messages to $PXF_LOGDIR/admin/pxf-cli-YYYY-MM.log
   • Log lines are prefixed with a timestamp and the PID of the shell script to help identify all log messages from a single invocation of the PXF CLI

4. Remove PXF CLI's dependency on curl command

   • the curl command was previously required because of its use in shutting down and checking the status of PXF

[ashuka24]

if this PR goes in, would https://github.com/greenplum-db/pxf/pull/831 be closed or would its contents need to be added to this PR?