Refactor set of routines specific to elog.c

This refactors the following routines and facilities coming from
elog.c, to ease their use across multiple log destinations:
- Start timestamp, including its reset, to store when a process has been
started.
- The log timestamp, associated to an entry (the same timestamp is used
when logging across multiple destinations).
- Routine deciding if a query can be logged or not.
- The backend type names, depending on the process that logs any
information (postmaster, bgworker name or just GetBackendTypeDesc() with
a regular backend).
- Write of logs using the logging piped protocol, with the log collector
enabled.
- Error severity converted to a string.

These refactored routines will be used for some follow-up changes
to move all the csvlog logic into its own file and to potentially add
JSON as log destination, reducing the overall size of elog.c as the end
result.

Author: Michael Paquier, Sehrope Sarkuni
Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/CAH7T-aqswBM6JWe4pDehi1uOiufqe06DJWaU5=X7dDLyqUExHg@mail.gmail.com

src/include/utils/elog.h

@@ -442,6 +442,18 @@ extern void DebugFileOpen(void);
 extern char *unpack_sql_state(int sql_state);
 extern bool in_error_recursion_trouble(void);
 
+/* Common functions shared across destinations */
+extern void reset_formatted_start_time(void);
+extern char *get_formatted_start_time(void);
+extern char *get_formatted_log_time(void);
+extern const char *get_backend_type_for_log(void);
+extern bool check_log_of_query(ErrorData *edata);
+extern const char *error_severity(int elevel);
+extern void write_pipe_chunks(char *data, int len, int dest);
+
+/* Destination-specific functions */
+extern void write_csvlog(ErrorData *edata);
+
 #ifdef HAVE_SYSLOG
 extern void set_syslog_parameters(const char *ident, int facility);
 #endif