ovsdb-idl: Optimize out transactions that are complete no-ops.

ovsdb-idl: Optimize out transactions that are complete no-ops.

Commit 1cc618c3252 "ovsdb-idl: Fix atomicity of writes that don't change a
column's value" fixed transactions that write the existing value to
some columns, ensuring that those columns still got written to the database
to avoid making the transaction nonatomic in the presence of writes that do
modify part of the database.

However, that commit was too conservative: we can still optimize out a
database transaction that writes *only* existing values to the database,
because if we drop such a transaction then the resulting database is still
one that could result from executing transactions in a serial order.  This
commit implements that optimization.

As an example of what this commit does, before this commit, an "ovs-vsctl
set" command that specified the existing value for a column would do a
round-trip to the database to write that existing value.  After this
commit, that round-trip would not occur.

Found by observing system startup.