Test solution for the feature request #473

[DarkStar1982]

Allows hot patching the new executable file into the same process using "execv" command

How to use:

0. run pgdog
1. run ps | grep "pgdog" (or pgrep "pgdog") to find its process id (PID)
2. run kill -USR1 [PID] will send SIGUSR1 to listener in pgdog to trigger "execv" command
3. watch the new executable starting up while keeping the current process id
4. run ps | grep "pgdog" or pgrep "pgdog" to confirm that process id didn't change

Repeat as many times as needed. You can use "cargo build" to create a new executable instance and hot-load it at any time into the same process.

[CLAassistant]

All committers have signed the CLA.

[levkk]

Hey, thanks for your contribution. The trick with this one is we have to restore pgdog to the same state it was before the restart:

1. Client and server connections must be maintained and be in the same state
2. Prepared statements cache must be maintained
3. Query cache, used for query routing, ideally should be maintained to avoid latency spikes

This requires serializing the state of the app and passing it down to the new process to "re-hydrate" the process.