Reduce memmoves in memory BIOs.

Currently, a read/write memory BIO pulls up the data via memmove() on each
read. This becomes very expensive when a lot of small reads are performed,
especially if there is a reasonable amount of data stored in the memory
BIO.

Instead, store a read offset into the buffer and only perform a memmove()
to pull up the data on a write, if we have read more than 4096 bytes. This
way we only perform memmove() when the space saving will potentially be of
benefit, while avoiding frequent memmove() in the case of small interleaved
reads and writes.

Should address oss-fuzz #19881.

ok inoguchi@ tb@

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