Log Message:
Update dovecot2-pigeonhole to 0.4.20.

Changelog v0.4.20:

+ Made the retention period for redirect duplicate identifiers
  configurable. For accounts that perform many redirects, the lda-dupes
  database could grow to impractical sizes. Changed the default
  retention period from 24 to 12 hours.
- sieve-filter: Fixed memory leak: forgot to clean up script binary at
  end of execution. Normally, this would merely be an inconsequential
  memory leak. However, when the script comes from an LDAP storage, this
  would cause io leak warnings.
- managesieve-login: Fixed handling of AUTHENTICATE command. A second
  authenticate command would be parsed wrong. This problem was caused by
  changes in the previous release.
- LDA Sieve plugin: Fixed minor memory leak caused by not cleaning up
  the sieve_discard script.