tx_gateway_frontend: expire all old txs instead of one at a time

Expiring one old txn sesison per new tx helps to maintain the txn
sessions cache size at the capacity but it won't bring the cache
size down if it's already beyond max_transactions_per_coordinator
(it may happen when a user sets max_transactions_per_coordinator
for the first time).

Fixing it by bulk expiring.

src/v/cluster/tx_gateway_frontend.cc

@@ -1147,20 +1147,25 @@ ss::future<cluster::init_tm_tx_reply> tx_gateway_frontend::limit_init_tm_tx(
 
         // similar to double-checked locking pattern
         // it protects concurrent access to oldest_tx
-        if (stm->tx_cache_size() > _max_transactions_per_coordinator()) {
+        while (stm->tx_cache_size() > _max_transactions_per_coordinator()) {
             auto old_tx_opt = stm->oldest_tx();
             if (!old_tx_opt) {
                 vlog(
                   txlog.warn,
-                  "oldest_tx shouldn't return empty when tx cache is at "
-                  "capacity");
+                  "oldest_tx should return oldest tx when the tx cache size "
+                  "({}) is beyond capacity ({})",
+                  stm->tx_cache_size(),
+                  _max_transactions_per_coordinator());
                 co_return init_tm_tx_reply{tx_errc::not_coordinator};
             }
 
             auto old_tx = old_tx_opt.value();
             vlog(
               txlog.info,
-              "tx cache is at capacity; expiring oldest tx with id:{}",
+              "tx cache size ({}) is beyond capacity ({}); expiring oldest tx "
+              "(tx.id={})",
+              stm->tx_cache_size(),
+              _max_transactions_per_coordinator(),
               old_tx.id);
             auto tx_units = co_await stm->lock_tx(old_tx.id, "init_tm_tx");
 
@@ -1188,7 +1193,7 @@ ss::future<cluster::init_tm_tx_reply> tx_gateway_frontend::limit_init_tm_tx(
             }
             tx_units.return_all();
         }
-
+        vlog(txlog.info, "tx cache size is reduced");
         init_units.return_all();
     }