prepared_statement: fix deserialization of compound partition keys

[piodul]

When PartitionKeyDeserializer wants to deserialize i-th column of the partition key, it needs to know the type of that column. It has access to PreparedMetadata of the prepared query the partition key was constructed from, which has a mapping between bind markers and types. Hence, it needs to learn to which bind marker the i-th pk column corresponds to.

The self.pk_indexes of type Vec<PartitionKeyIndex> is supposed to help with that. The PartitionKeyIndex structure looks like this:

pub struct PartitionKeyIndex {
    /// index in the serialized values
    pub index: u16,
    /// sequence number in partition key
    pub sequence: u16,
}

So, we need to find an entry pki with pki.sequence == i and pki.index will be the desired bind marker index.

Currently, we naively look at the self.pk_indexes[i] entry - which is wrong. The pk_indexes vec is actually ordered by index, so we might accidentally obtain an index of the wrong partition key column this way. Although there was a test that was supposed to check this scenario, it constructed PreparedMetadata manually and didn't sort the pk_indexes by index field, which resulted in it accidentally passing.

In order to fix this, the PartitionKeyIndex is now doing a linear lookup in order to find a pki with pki.sequence == i. This lookup should be fine as partition keys usually have a little number of fields.

Fixes: #793

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