On macOS, add a check for memory available

[bitcoin-tools]

https://apple.stackexchange.com/questions/4286/is-there-a-mac-os-x-terminal-version-of-the-free-command-in-linux-systems

[bitcoin-tools]

user1@comp4 nodebuilder % sysctl hw.memsize
hw.memsize: 19327352832

[bitcoin-tools]

user1@comp4 nodebuilder % memory_pressure       
The system has 19327352832 (1179648 pages with a page size of 16384).

Stats: 
Pages free: 279933 
Pages purgeable: 86796 
Pages purged: 5341286 

Swap I/O:
Swapins: 53612 
Swapouts: 77276 

Page Q counts:
Pages active: 296553 
Pages inactive: 191729 
Pages speculative: 102305 
Pages throttled: 0 
Pages wired down: 157016 

Compressor Stats:
Pages used by compressor: 98040 
Pages decompressed: 59403359 
Pages compressed: 60691384 

File I/O:
Pageins: 12486878 
Pageouts: 78609 

System-wide memory free percentage: 77%
user1@comp4 nodebuilder % 

[bitcoin-tools]

https://apple.stackexchange.com/questions/423717/is-there-a-command-line-tool-that-accurately-describes-the-amount-of-used-memory