Reserve max buffer size in delta performer.

From the analysis in b/34220646#comment68, repeatedly insertion to the
vector caused memory reallocation and potentially increase the peak
memory usage a lot. This issue can be easily fixed by reserving the
memory first since we know the patch size in advance.

Bug: 34220646
Test: run an incremental update on sailfish and check the memory usage.
Change-Id: I89726cc5c8d51c2e43ac8616c711eeabc3f88dbc