MDEV-34431: Excessive blocking by mtr_t::index_lock_upgrade()

[dr-m]

mtr_t::index_lock_upgrade(): If a non-blocking upgrade of dict_index_t::lock to exclusive is not possible, release and reacquire the latch, to give concurrent threads a better chance to run.

ssux_lock_impl::u_wr_upgrade_try(), sux_lock::u_x_upgrade_try(): A new operation, to try non-blocking upgrading from U to exclusive.

sux_lock::s_x_upgrade_try(): Make use of u_x_upgrade_try() in order to implement fully non-blocking operation.

[CLAassistant]

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