cel/cel_odbc: Provide microsecond precision in 'eventtime' column when possible

This patch adds microsecond precision when inserting a CEL record into a table
with an "eventtime" column of type timestamp, instead of second precision. The
documentation (configs/cel_odbc.conf.sample) was already saying that the
eventtime column included microseconds precision, but that was not the case.

Also, without this patch, if you had a table with an "eventtime" column of
type varchar, you had millisecond precision. With this patch, you also get
microsecond precision in this case.

Review: https://reviewboard.asterisk.org/r/3980

ASTERISK-24283 #close
Reported by: Etienne Lessard
patches:
  cel_odbc_time_precision.patch uploaded by Etienne Lessard (License 6394)
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Merged revisions 427952 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 427953 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@427954 f38db490-d61c-443f-a65b-d21fe96a405b