app_voicemail: Fix sql insert mismatch caused by cherry-pick

app_voicemail: Fix sql insert mismatch caused by cherry-pick

When commit e8c9cb80 was cherry-picked in from master, the
fact that the 20 and 18 branches still had the old "macrocontext"
column wasn't taken into account so the number of named parameters
didn't match the number of '?' placeholders.  They do now.

We also now use ast_asprintf to create the full mailbox query SQL
statement instead of trying to calculate the proper length ourselves.

Resolves: #831
(cherry picked from commit 83b378349945ba5de0c2dfdc19a7dd4c6aed0102)