Make some erc-stamp functions more limber

Make some erc-stamp functions more limber

* lisp/erc/erc-stamp.el (erc-timestamp-format-right): Deprecate option
and change meaning of its nil value to fall through to
`erc-timestamp-format'.  Do this to allow modules to predict what the
right-hand stamp's final width will be.  This also saves
`erc-insert-timestamp-left-and-right' from calling
`erc-format-timestamp' again for every inserted message.
(erc-stamp-mode, erc-stamp-enable, erc-stamp-disable): Add
`erc-stamp--recover-on-reconnect' to `erc-mode-hook'.
(erc-stamp--recover-on-reconnect): Add function to reuse last values
of `erc-timestamp-last-inserted' and friends to avoid reprinting
stamps when reconnecting.
(erc-stamp--current-time): Add new generic function and method to
return current time.  Default to calling `erc-current-time'.  Also add
new internal variable of the same name to hold time value used to
construct formatted stamps passed to `erc-insert-timestamp-function'.
(erc-add-timestamp): Bind `erc-stamp--current-time' when calling
`erc-insert-timestamp-function'.
(erc-stamp--insert-date-function): New variable for modules to tweak
date-insertion.
(erc-insert-timestamp-left-and-right): Use STRING parameter and favor
it over the now deprecated `erc-timestamp-format-right' to avoid
formatting twice.  Also extract current time from the variable
`erc-stamp--current-time' for similar reasons.  And defer to
`erc-stamp--insert-date-function' to insert left stamp.  (Bug#60936.)
(erc-stamp--tz): New internal variable.
(erc-format-timestamp): Pass `erc-stamp--tz' as time-zone to
`format-time-string'.