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connpool: prefer_ro and prefer_rw for call()

This patch introduces "prefer_ro" and "prefer_rw" values for the "mode"
option for the "call()" function.

Closes #9930

@TarantoolBot document
Title: The `mode` option for `call()` and `filter()`

The new `mode` option is now supported by the `call()` and `filter()`
functions from the `experimental.connpool` module. This option allows to
filter candidates based on their `read-only` status.

The `filter()` function supports three values of the `mode` option:
1) `nil` means that the `read_only` status of the instance is not
   checked;
2) `ro` means that only instances with `read_only == true` are
   considered;
3) `rw` means that only instances with `read_only == false` are
   considered.

The `call()` function supports five values of the `mode` option:
1) `nil` means that the `read_only` status of the instance is not
   checked when instance is selected to execute `call()`;
2) `ro` means that only instances with `read_only == true` are
   considered when instance is selected to execute `call()`;
3) `rw` means that only instances with `read_only == false` are
   considered when instance is selected to execute `call()`.
4) `prefer_ro` means that `call()` will only be executed on instances
   with `read_only == false` if it is not possible to execute it on
   instances with `read_only == true`;
5) `prefer_rw` means that `call()` will only be executed on instances
   with `read_only == true` if it is not possible to execute it on
   instances with `read_only == false`.

Note that if this option is not `nil`, a connection will be attempted to
each instance in the config if a connection does not exist. This means
that any of these functions can potentially block for a maximum of
`<number of instances> * 10` seconds.

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