app_voicemail_odbc: Allow audio to be kept on disk - 18,20

[gtjoseph]

This commit adds a new voicemail.conf option 'odbc_audio_on_disk'
which when set causes the ODBC variant of app_voicemail to leave
the message and greeting audio files on disk and only store the
message metadata in the database. This option came from a concern
that the database could grow to large and cause remote access
and/or replication to become slow. In a clustering situation
with this option, all asterisk instances would share the same
database for the metadata and either use a shared filesystem
or other filesystem replication service much more suitable
for synchronizing files.

The changes to app_voicemail to implement this feature were actually
quite small but due to the complexity of the module, the actual
source code changes were greater. They fall into the following
categories:

• Tracing. The module is so complex that it was impossible to
  figure out the path taken for various scenarios without the addition
  of many SCOPE_ENTER, SCOPE_EXIT and ast_trace statements, even in
  code that's not related to the functional change. Making this worse
  was the fact that many "if" statements in this module didn't use
  braces. Since the tracing macros add multiple statements, many "if"
  statements had to be converted to use braces.

• Excessive use of PATH_MAX. Previous maintainers of this module
  used PATH_MAX to allocate character arrays for filesystem paths
  and SQL statements as though they cost nothing. In fact, PATH_MAX
  is defined as 4096 bytes! Some functions had (and still have)
  multiples of these. One function has 7. Given that the vast
  majority of installations use the default spool directory path
  /var/spool/asterisk/voicemail, the actual path length is usually
  less than 80 bytes. That's over 4000 bytes wasted. It was the
  same for SQL statement buffers. A 4K buffer for statement that
  only needed 60 bytes. All of these PATH_MAX allocations in the
  ODBC related code were changed to dynamically allocated buffers.
  The rest will have to be addressed separately.

• Bug fixes. During the development of this feature, several
  pre-existing ODBC related bugs were discovered and fixed. They
  had to do with leaving orphaned files on disk, not preserving
  original message ids when moving messages between folders,
  not honoring the "formats" config parameter in certain circumstances,
  etc.

UserNote: This commit adds a new voicemail.conf option
'odbc_audio_on_disk' which when set causes the ODBC variant of
app_voicemail_odbc to leave the message and greeting audio files
on disk and only store the message metadata in the database.
Much more information can be found in the voicemail.conf.sample
file.

[gtjoseph]

cherry-pick-to: 18