reduce megolm sync cache size to 1 session

as this is kept around for every e2ee room, and we only have
limited olm memory

src/matrix/e2ee/RoomEncryption.js

@@ -41,7 +41,7 @@ export class RoomEncryption {
         this._encryptionParams = encryptionParams;
 
         this._megolmBackfillCache = this._megolmDecryption.createSessionCache();
-        this._megolmSyncCache = this._megolmDecryption.createSessionCache();
+        this._megolmSyncCache = this._megolmDecryption.createSessionCache(1);
         // session => event ids of messages we tried to decrypt and the session was missing
         this._missingSessions = new SessionToEventIdsMap();
         // sessions that may or may not be missing, but that while

src/matrix/e2ee/megolm/Decryption.js

@@ -41,8 +41,8 @@ export class Decryption {
         this._olmWorker = olmWorker;
     }
 
-    createSessionCache(fallback) {
-        return new SessionCache(fallback);
+    createSessionCache(size) {
+        return new SessionCache(size);
     }
 
     /**

src/matrix/e2ee/megolm/decryption/SessionCache.js

@@ -14,13 +14,14 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-const CACHE_MAX_SIZE = 10;
+const DEFAULT_CACHE_SIZE = 10;
 
 /**
  * Cache of unpickled inbound megolm session.
  */
 export class SessionCache {
-    constructor() {
+    constructor(size) {
+        this._size = typeof size === "number" ? size : DEFAULT_CACHE_SIZE;
         this._sessions = [];
     }
 
@@ -51,12 +52,12 @@ export class SessionCache {
         sessionInfo.retain();
         // add new at top
         this._sessions.unshift(sessionInfo);
-        if (this._sessions.length > CACHE_MAX_SIZE) {
+        if (this._sessions.length > this._size) {
             // free sessions we're about to remove
-            for (let i = CACHE_MAX_SIZE; i < this._sessions.length; i += 1) {
+            for (let i = this._size; i < this._sessions.length; i += 1) {
                 this._sessions[i].release();
             }
-            this._sessions = this._sessions.slice(0, CACHE_MAX_SIZE);
+            this._sessions = this._sessions.slice(0, this._size);
         }
     }