Ensure sequences are replicated even if changed in de-facto read-only transactions

[XaBbl4]

Add a new method to the replication API to ensure generators are always replicated (both on commit and rollback).

Previously, if execute:

select gen_id(t_gen, 1) from rdb$database;
rollback;

The generator on the master and replica stored different values.

If decide to backport this to 5.0, might consider:

version: // 5.0 => 5.0.4
    void flushSequences(Status status);

version: // 5.0.4 => 6.0 Alpha1
    void setSequence2(...);

[dyemanov]

In practice this PR avoids inconsistent replica when applications use an attachment-level sequence cache (IDs are prefetched and distributed among rows being inserted). This does not allow sharing the cached IDs among different attachments, but so far we haven't seen such an use case.

Custom replication plugins are free to choose between synchronous or delayed (conditional or unconditional) sequence replication.