MINOR: http-act: Add 'pause' action to temporarily suspend the message analysis

MINOR: http-act: Add 'pause' action to temporarily suspend the message analysis

The 'pause' HTTP action can now be used to suspend for a moment the message
analysis. A timeout, expressed in milliseconds using a time-format
parameter, or an expression can be used. If an expression is used, errors
and invalid values are ignored.

Internally, the action will set the analysis expiration date on the
corresponding channel to the configured value and it will yield while it is
not expired.

The 'pause' action is available for 'http-request' and 'http-response'
rules.