Enhance the pcache1 page cache so that it tries to allocate a block of

Enhance the pcache1 page cache so that it tries to allocate a block of
SQLITE_DEFAULT_PCACHE_INITSZ pages from malloc() on startup, and uses those
preallocated pages when possible rather than going to malloc() for each
individual page.  About a 5% performance increase for some workloads.

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