pgsql: Add value 'current' for recovery_target_timeline

Add value 'current' for recovery_target_timeline

This value represents the default behavior of using the current
timeline.  Previously, this was represented by an empty string.

(Before the removal of recovery.conf, this setting could not be chosen
explicitly but was used when recovery_target_timeline was not
mentioned at all.)

Discussion: https://www.postgresql.org/message-id/flat/6dd2c23a-4162-8469-410f-bfe146e28c0c@2ndquadrant.com/
Reviewed-by: David Steele <david@pgmasters.net>
Reviewed-by: Michael Paquier <michael@paquier.xyz>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ff85306055ede1fbca86ca4dfc4d48172342462b

Modified Files
--------------
doc/src/sgml/config.sgml                      | 8 +++++---
src/backend/utils/misc/guc.c                  | 4 ++--
src/backend/utils/misc/postgresql.conf.sample | 3 +--
3 files changed, 8 insertions(+), 7 deletions(-)