[SalesForce] Possible to use runAs in Apex Trigger

Instead of calling a single @future(callout=true) method you can start a batchable running where the number of records processed in each execute call can be limited to be below the governor limit:

public class Plexure {

    public static void PushMessage(Set<Id> pushMessageIds) {
        Database.executeBatch(new MyBatchable(pushMessageIds), 50);
    }
}

A batchable breaks work for many records up into multiple limited size lists of records. Your TMP_Push_Messages__c query goes into the start method and the rest of the code goes into the execute method.

You also need to add the Database.AllowsCallouts marker interface:

public class MyBatchable implements Database.Batchable<sObject>, Database.AllowsCallouts {

    private Set<Id> pushMessageIds;

    public MyBatchable(Set<Id> pushMessageIds) {
        this.pushMessageIds = Set<Id> pushMessageIds;
    }

    public Database.QueryLocator start(Database.BatchableContext bc) {
        return Database.getQueryLocator([
            select consumerId__c, messageId__c
            from TMP_Push_Messages__c
            where Id IN :pushMessageIds
        ]);
    }

    public void execute(Database.BatchableContext bc, List<TMP_Push_Messages__c> scope) {
        ...
    }

    public void finish(Database.BatchableContext bc) {
    }
}

If you don't care about the delete contexts, you can just do:

SObjectType triggerType = trigger.new.getSObjectType();

It doesn't add much complexity, however, to use a ternary to check for the null case:

SObjectType triggerType = trigger.isDelete ? 
    trigger.old.getSObjectType() :
    trigger.new.getSObjectType();