Use growth factor 1.5x for `read_buffer`

[vsoulgard]

Change the buffer growth strategy in read_buffer::grow_to_fit() from allocating exactly required_size to allocate std::max(required_size, buffer_.size() * 1.5) or required_size * 1.5, because:

1. This could reduce amount of reallocations (imagine situation where reallocation happens just for few bytes).
2. This could reduce heap fragmentation in case we use std::max(required_size, buffer_.size() * 1.5), because we amortize future reallocations to match growth factor of 1.5.

But memory usage will be higher. In the worst case, the buffer capacity may grow to 1.5× the maximum message size, but that also mean we can avoid memmoves for such big messages if we use approach mentioned in #501

[anarthal]

This sounds reasonable to me. Being buffers, I could even see making buffer sizes always powers of two. We need to take into account the memory limit configured by the user when reallocating, too.